The pop cap bug was fixed years ago, so the two first migrant waves respect the pop cap. Thus, a pop cap of 1 -7 will block migrants (with 7 you're going to eventually get losses recovered, which I suspect you don't want). Actually dead civs won't generate any migrants beyond the two first waves, so if you block out those you won't get any replacements, but you should probably use a pop cap of 1, as there's no point in a higher number for a no migrant fortress.
Void dorf can be married to each other, and they can be a parent and a kid (and probably both parents and several kids if the RNGs align).
It can be noted, though, that breeding a fortress from the starting 7 is doomed fairly frequently, as age, gender distribution, and sexual preferences, coupled with personality, can frequently lead to a starting 7 team which doesn't have two compatible pairs (which is required for a second generation). It's possible to work around this by backing out from embarking and restarting if examination of the 7 while on the embark purchase screen shows a poor cast.
I play dead civs, and receive the two starting waves, resulting in a pop ranging from 12 to 22 or so, and I tend to manage to cobble together 2-5 couples, provided the starting 7 passes a basic pre embark screening (6 of 1 gender and 1 of the other isn't a good start...).