You can, with a few tweaks in worldgen, create a world that only has dwarves and goblins in it.
Further, you can create a world that has exactly one dwarven civ in it, with many goblin civs, and cause-to-be-extinct the single dwarven civ.
In the end, though, it doesn't really matter. Civ or extinct civ, you'll get dwarves. As many as you need, concocted out of Armok's Ether on demand.
To get as few dwarves as possible, remove: [ACTIVE_SEASON:AUTUMN] from [ENTITY:MOUNTAIN] in raw/objects/entity_default.txt
This will prevent liaisons and caravans from ever visiting your site. You'll get two waves of migrants, and that's it.
Unfortunately, it can also create a strange problem, whereby your fort is not considered "ready" for invaders, in particular if you never go above 20 dwarves. 20 is the magical number. If you get to 21+, everything will work as expected. thieves, babysnatcher, abushes, sieges. If you reach EXACTLY 20, things can go a little wonky/strange. If you never reach 20, you may never get invaders.
The way to tell if things are working right is, if by spring of the second year (so you've gone through, spring, summer, autumn, winter, spring since embark) and you create a job with the manager, and it has to be validated? Everything is working properly. If you get to that point in time, and manager jobs are still queue'd instantly? You're boned. Things will be wonky/strange in that fort, forever.
If you get to exactly 20 dwarves (from the first two hardcoded migrant waves) before the outpost liaison and first caravan shows up? They'll never show up. No matter what you change for settings. And likely, your outpost will not be considered ready for invaders, either.
Unfortunately, after going through this many times with experiment forts, my solution has been to turn off invaders in d_init.txt, set the pop_cap to 22 to ensure I get 20+ migrants, allow the first caravan/liaison to arrive, THEN remove the [ACTIVE_SEASON:AUTUMN] from the save raws, and wait until all the remaining migrant waves get to the fort, and the "the fort attracted no migrants this year" message comes up for two seasons in a row. After all that is done, I turn invaders back on, and the goblins come forth as expected.
Having seen all the permutations of this particular mechanic, I can see why Toady wants to change migration in general. (as he mentioned in the last
Dwarf Fortress Talk.)
Yeah, and there'll be situations where that group doesn't exist, and then you probably wouldn't expect the migrant waves at all, in that case, or you might occasionally get some, but it wouldn't be like it is now. So everything's going to change, that's basically the bottom line on migrants, is that everything is going to change.