My fortress has reached a critical mass of about 110 dwarves. Since I'd like my FPS to stay above 50, pretty much all new migrants are unwanted. However, I've thought of a fun way to put the migrants to use instead of killing them outright.
My main fortress military consists of 20 dwarves: 10 steel-clad axedwarves, and 10 marksdwarves.
What I plan to do is draft all new migrants into the military, and place them in a burrow separate from the main fortress, underneath and connected to a watchtower on the countryside.
I plan on using them in order to keep watch over my surface workers and deal with (or at least delay) any ambushes. They will be my first-response teams to any threat on the surface.
I just don't want to deal with the headache of producing steel equipment for the 30 or so dwarves that will make up this rag-tag militia, so they will be poorly equipped. I will provide some equipment, but they will mainly be using whatever weapons and armor they can scavenge from the goblins. I don't really care for their combat survivability anyway, since this is an alternative to killing the migrants straight away.
Who knows, maybe some of the dwarves will prove their worth in combat and join the ranks of the fortress elite. Most of them will probably die. Either way, they're put to good use.