I always start off with a max cap of 30, with babies and invasions turned off. Based on what I'm trying to do with that individual fort, I assign labors to the immigrants based (hopefully) on what their actual skills are, but if their skills are useless (fishery jerks) or lower than all the other dwarves already in that industry (the bottom of the totem pole, as it were), then I just assign them any jobs that still need filling, at least until someone else comes along with better stats for it.
Once the pop is a bit fleshed out, labors are somewhat distributed, and the fort is starting to develop, I crank the pop cap up to 49 - this means that I usually get a nice chunk of migrants in the next wave (enough to fill out any industries that still need it), nobody gets to make annoying mandates yet, and any superfluous female dwarves either get assigned jobs as "apprentice" (any jobs that anybody can do, or that I don't want my actual industry dwarves wasting time on), while the extra men get set up as the first batch of recruits in training for military service.
Once all the industries are staffed, the workshops built, the fledgling military in training, and the fortress is finally coming together, I turn on babies, I turn on invasions, and I crank the population up to the "final destination" - whatever population i want toe fort to ultimately support. In some cases, this is only about 60, but once in a while, I feel like having more dwarves for military or projects, so I might play with anywhere from 90 to 120.
I haven't played with a fort larger than that for a LONG time, since I find that a well-managed group of 100+ dwarves is just as effective (if not moerso) than a hodge-podge band of 200.
I'm currently toying with the idea of just setting the cap at 150ish and allowing infinite babies. It'll be a madhouse, but I've been playing with carefully plotted out population controls for so long, I figure it's about time to throw caution to the wind and enjoy the game like a newbie again.