You need to have patience, for the DF community isn't that big. Truly, game's fame far exceeds its grasp.
Also, large question. PatrikLundell posted before I did, so there's some overlap...
Now, as for your question, I don't know everything - I have barely modded atm - but to answer you...
⛭
Starting population for civ placement is 100 for each of the civs, expect for dwarves, who start with 200.
The minor sites (hillocks, small forest retreats, dark pits, hamlets) are usually around 100ish population, which I presume is controlled by
[MAX_SITE_POP_NUMBER:120]
in entity.default. However, I've seen thousands - up to 4200 - in minor sites on rare occasions. Not certain what causes those.
The major sites are a little more complicated. First, some attributes shared by major sites:
• Past civ placement phase of worldgen, you can't establish a major site in 7 tile range of another major site, even if the first major site is destroyed.
• Major sites get access to the resources of all subordinate sites oof their range. Their markets have better, greater variety of choices. Though I don't think goblins sell anything....
• They're a target for immigration of animal people/proto-civilization joining. Large biomes give more animal people, and they can do so across the oceans. Though goblins only get their pets.
Ω - (dwarven) fortress. Seems to have cap of 200 dwarves in it, but libraries and joining animal men/protocivilizations can inflate your world's fortresses sites and pops .txt waaaaay past that. Most common seem to be gorlaks and plump helmet men, of which I've seen thousands of each.
They also often die relatively quickly to FBs (within a century) in worldgen unless they get to build a hillock nearby, which will repopulate them after a rampage. After hillocks are built they can further build dwarven mountain halls Ω, which are entirely underground settlements.
Dwarves always start with a major site - very convenient for restraining human towns and elven retreats.
R - (goblin) dark fortress (R is closer to curses cabinet than pi symbol).
Usually gets a structure of 10k goblins and 2527 "pets" - beak dogs, trolls and ogres. This takes around 2-400 years and can even occur if the site is completely isolated - say, by being placed in the middle of evil area with 7 tiles of terrifying plains or ocean all around.
It is not just the immortal, sleepless, not thristy or hungry goblins that give these their power. Being led by a demon leader is a considerable boon, which allows them to usually survive and thrive even in megabeast-heavy worlds; unlike the other races.
Should the site be conquered - by, say, elves with giant elephants - the resultant site will still be able to have 10k civilized population, expect it might now be 10k elves.
Goblins always start with one, much like dwarves. Though they can be put in much wider variety of locations.
¶ - Elven forest retreat.
The civilized population, while inconsistent, seems to range from 400 to 3000 for longer-term worlds. I'm not sure of the mechanics.
Elven forest retreats tend to have numerous animals available, though some get capped at 10 or 35 pop. Others rise with population much like the goblin pets, which means hundreds for ¶.
+⁎#⛭ - human town.
Probabably most complicated site, the town is built in two phases - first, the keep, and then, the sewers.
Keeps typically have 500-1500 population and + or ⁎ symbol.
Once the sewers are built, the population explodes into # or ⛭, which have 10k population cap much like R. This can happen from breeding, but if it took a bit the immigration is going to be main factor. If you have two isolated islands, one getting a dark fortress and other a town, then the time the sewers are estabilished is going to be only time dark fortress population drops by the thousands, resulting in partially-goblin race town. Elves and dwarves can immigrate too, but they won't be doing it in so great numbers.
With first keep, you also get the first temple in the world, which means the start of werebeasts and vampires - unless you set the numbers of those to 0 in advanced worldgen. Only towns get temples.
A vampire, or a cabal of vampires, being one of the few things that can keep the lid on town's population. If your town is struggling and goes to + from #, it's likely there's at least one vampire with up to tens of thousands of kills present.
❄ New sites.
Honestly, if you have unsavage suitable biome nearby with no adjacent tower, vault or other such thing blocking it, expect them to build one if they can breed.
Unless, you set the site cap in advanced worldgen to some positive number. This has the effect of preventing the founding of new sites (including towers) once that number of sites is reached). Civilizations will still be placed if you, say, set it to 1 with 4 civs, but they'll never make towers or smaller offshoots in that case. There is also a max civ pop, but I'm not sure what effect that has.
As sites reduce savagery near them, I find it better to restrain site building with pre-set values. Here's some numbers:
30: Calm. Given 0, 10, 20 and 30, human civs in civ placement phase tend to be placed on 30 calm plains the most for some reason.
67: Not going to place a civ here, but when a site gets nearby it is eligible shortly.
73: Take a bit of time to break through, around 400-700 years though can be up to 1200 on rare occasion (you'll probably get them to conquer another site and build from there sooner if they have the range).
90: Next millennium is a likely possibility. Up to 1800 years, though you're more likely to see half that.
100: Take up to several millennia to break through, if this is all you have to work with from a dinky hamlet.
Bigger and multiple ones reduce faster. Also, elves don't reduce the savagery of joyous wilds, and can settle on there fine - use partial taiga instead if you need a large forest region for good biome.
o Death.
Your small sites may not just be attacked by goblins - they may be at war with others at the time, or even. themselves.
Need for food and death reduces site populations. Childs also do not reproduce, but old men/women do. [BONECARN] is a tag that limits creature to eat just meat, which adds a complication to Kobold Camp.
Can't say much on the effects of orientation. All usual rules, such as age gap, are not followed. You can have werebeasts who give birth to themselves and then get cursed, though usually the marriage waits until they're an adult.
Instead of littersize, you could use egg laying. Kobolds do manage to reproduce, so it could give you more granular control, though fortress mode would be a hassle to manage.
Yeah, the game has a pregnancy notion. Sentient beings are pregnant for 9 months, non-sentient 6. Birds lay fertile eggs for that duration instead, which take a season to hatch. I'm not sure pregnancy duration is tracked for histfigs, though - I've seen no miscarriage in legends, and can't find a data structure for it.