1. They should be big enough to trade - ~100 is the size of your typical civ at the dawn of time, and you can embark in year 5 with even basic world gen.
However, what civilization can bring in trade depends on what it has met in worldgen. The scholarly and taverny arts in particular need some time to learn. I included terrifying plains for the chance of goblins getting ogres off them, but for variety of brought animals and plants maximising different bordering biomes is necessary for variety of those brought in trade goods. Dwarves similarly get access to more minerals, if they expand more. No idea about the quality of goods, however.
War, however, is somewhat different story.
A civilization only sends out attacks from a single site only (even after that site gets conquered, they still send from there), and that single site doesn't grow fast/pull population from elsewhere. Furthermore, while it is not typically looked at much (legends mode requiring a retire and all), I do recall only a single case where a succession fortress was besieged by 2 different goblin civilizations. Still, you usually get attacked by only 1 civ of a species.
You may get besieged by all the different towers in range, each being their own civ, but they similarly don't have much more population than your typical hamlet - once they're killed instead of retreating, the dead no longer walk. Put together, the above example might have around 2000?? undead to siege you.
Not just any site can grow big. After the initial 100ish, population growth is mostly funneled to ¶, # and
Π sites which cap out at around 10-13k sentient population(in few centuries). Since elves and humans can get fucked, the population of such big sites may very well be majority of nationalized goblin immigrants.
Goblins, thankfully, can grow big from just initial starting site with no expansion, then build new one in 8, then 14 tiles. Furthermore, as you can see above, megabeasts are unlikely (well, it can happen sometimes) to attack goblin dark fortress, giving them unparalleled growth opportunities.
Humans usually, but not always, grow a city in their second site (can also be first or third regularly, but they do need some additional sites), then grow new cities every ~7 tiles (sometimes leaving a gap - depending on terrain?, though).
Elves are a mix of two, building a new huge retreat in 7 or 13 tiles. Like humans, it is consistent but random-seeming where they'll do the population centers.
Dwarves never do anything big. If you want them to get numbers, you need to give them plains to expand on.
Additionally, the new site must be suitable for placement, to a bit more stringent requirements than your usual worldgen: Savagery limits become more of a rule than guideline. Goblins don't wear down the savagery of evil areas, allowing limiting them to dark fortresses. Otherwise, civ has to drop down the savagery of surrounding suitable biomes over centuries, or even millennia.
Oh, and they must not get killed, of course.
2. They're pre-set world values. Each number matches the base value of a single of six categories(elevation, rainfall, drainage, temperature, savagery, volcanism) given to a single region tile. Evil and good can be shepherded as well with limiting their desired areas to only small, medium or large region (thus the small evil plain surrounded by large good forest). I use all but temperature in the above, but if volcano were to be dropped volcanism could be as well.
The native interface is world painter. vjek, naturally, has a
tutorial.
Myself, I used a worksheet to create the above for precise control. There is also a program called
Perfect World DF for recreating landscapes, real, random or imagined.
Since civilizations are pretty much region tile features this allows high degree of precision in their placement. Embark tile level is still something of a RNG, though.
3. Heh. That's because it is a cage match. The above example is ~1/3 ruined sites, but still has 9 dragons, 9 hydras, 8 rocs, 5 bronze colossi: 31 of 40 starting megabeasts.
Evil prevents placements of most civilizations, but doesn't usually eat them, unlike dragons. (I once saw an elven civilization get mostly wrecked by zombie ogre rampages across the ocean from a tiny isthmus, though. They do sometimes take minor damage from untamed wilds they're on, as well.)
I picked elves as meat shields because:
a) goblins usually don't get problems with megabeasts (hm, maybe the demon masters killed those 9?)
b) goblins can be placed in pretty much any non-good area on world gen, so long as there are demons - and only elves can settle in good areas.
c) the worldgen rotates through civilizations to place them in roughly equal ratios usually, though starts to skips them if it is no longer unable to place that type of civilization.
206 civilizations is obtained by pre-controlled savagery - mountains and outer plains are calm, while good forest and central plain are untamed. Overall, almost the whole world is settleable, so 206 isn't the limit, just what I picked to be safe. (The actual limit is squishy on account of random layout of corner mountains and what embark tiles the civ sites overlap with.)
Without said pre-set values, the overall result won't look anything like what I did - heck, I didn't even limit the min-max elevations to actually used 160-330 range.
A mid-point might be possible, perhaps. If humans/elves have enough time and space to survive and build a +/¶, there is possibility of using isolated goblin islands to populate these sites with immigrants (although they might go to dwarven library instead). With enough immigrants, they would perhaps stand a chance of getting one capable of killing megabeasts instead of being killed by them.
There are some hidden assumptions, though: For fortress mode, though, you want to guarantee that their city is closest, and I have no idea how "first to meet" is decided with multiple species.
Furthermore, wars are generally fought with those who don't share the values. One way to block out wars and guarantee that they're the only civ of that species in range would be using ocean-isolated islands. However, this would likely also block out towers. And if the towers get placed on the islands, they may go to war regardless. (Towers, however, are the only civ that can get placed on a good taiga, so this can be reduced.)
Region tiles with multiple ocean shores can act as bridges to civ ranges when placing the fort. (I do recall there was some bug with visitors being prevented by 2+ ocean shores in a region, though.)
With a city, retreat, dark fortress and towers desired, you're looking at 3 or 4 shores in a single region. Due the 2-dimensional nature of generated worlds, this alone can easily bring the success rate for a single contact point below 20%. Additionally, even using a terrifying ocean and 2-tile islands (not sure if this is enough for expansion), you'll at best get only a single river that is much more likely to flow into one of the side squares, not into the desired corner.
PatrickLundall might have a gen for that, though probably with less megabeasts.
Speaking of which, there is a third hidden assumption: How many megabeasts alive and in range do you desire?
Consider: Even in a pocket world with 2000 megabeasts, you don't get attacked by more than one at a time unless you settle on an overlapping lair, iirc.
And speaking of range, megabeasts also share the 20 tile range restriction that civilizations have for attacking you, and are placed randomly on ground as well. Towers have 10 tile range, iirc. Thus, for most practial purposes, there is no little difference between a 129 gen and 33 gen repeated 16 times, or 17 gen repeated 64 times.
If I mostly just expanded the cup 8 times on each dimension with corresponding 13184 civilizations and 4480 megabeasts+titans, you would be unable to get humans in range in the central evil area due them all being 24-64 tiles away, assuming you wouldn't run out of RAM with over 1648000 sentients needing to be stored (if stored alongside creatures would certainly go into tens of millions).
Don't think that's what you want, though.
(Not all purposes, though. You can only get dry broadleaf forest in a world with a pole that is at least 129 tiles tall, for instance.)