I've tried this before in the first cavern layer. I've actually found that specifying two dwarves as military and having them train non-stop rather than having a 7-dwarf militia tends to work better, because they quickly outclass any monsters that happen to wander in, and with only 7 dwarves you don't need to put much work into getting food and booze, and you can have miners be masons and carpenters (but not the fort woodcutter, so a nice job for him is doubling as an herbalist).
If you aren't leaving migrants out of the fort, than you can pretty much expand your workforce the same way you usually would, but probably are going to want to divide your military into squads of 3 so that you can defend multiple parts of your fort from wild animals. If you aren't going to let them in, you really need to kill them off rather than just leave them because they will survive indefinitely on river water and vermin, until one that actually has a weapon goes berserk, and even then they don't ever die. Keep in mind you'll have a lot of memorializing to do.
Another thing to note, current research has concluded that dwarves will not mate with first cousins, but will mate with aunts and uncles. So, if you aren't taking migrants, know that there is hope for your second and third generation if they end up being fairly small.