Generating a world with a very small dwarf population IS feasible, and it is logical and probably true that abandoning multiple fortresses with a civ of otherwise very low population will result in higher and higher ratios of migrants from old forts to migrants never seen before in your forts. Especially children and babies. There is a
succession game based entirely around building many, many forts and abandoning them to various fates for the purposes of making the world more interesting for adventurers, and overseers do regularly receive migrants from older forts. Legendary miners that show up at your fort after zero effort from you are great...
There is also the
Museum Adventure Item Quest game, which has a similar thing going on. I played recently, and got quite a few historical figures from old forts, who promptly died there. I hope nobody misses all those artifact makers that immigrated to my fort.
Try generating a world with only one dwarven civ (I think you can do this by going into the raws, finding entity_default.txt, and under the MOUNTAIN civ definiton (the first one in the file) change [MAX_STARTING_CIV_NUMBER:100] to [MAX_STARTING_CIV_NUMBER:1]. That usually prevents the game from creating more than one civ of a specific definition. You can try changing;
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
to;
[MAX_POP_NUMBER:25]
[MAX_SITE_POP_NUMBER:50]
as well, but last I tried fiddling with those tags they didn't do anything at all, and the civ's growth was not stunted. Besides that, there's not much you can do to guarantee a low population besides make goblins more terrifying (in the hopes that they will beat down the other civs more effectively) and increase the titan, mega- and semi-megabeast populations significantly. (Also hoping that they will beat the crap out of the dwarven population.)