Worth noting concerning the pop cap feature: you can currently expel dwarves from your fortress on a whim, so that should be the ideal technique for now.
Is it possible to have dwarves homestead a hillocks for now, or is claiming or embarking near a hillocks all that can be done presently? I believe there was also a feature to send dwarves "out to pasture" at a hillocks, anf to bring them back too.
Existing hillocks will link themselves to you economically once you build up some wealth. In addition your civilization will found hillocks near your site in the same way. You can send dorfs there and call them back with a messenger. They tend to be just s stressed as when you first sent them...
Or you can send them to some place you've conquered, or just send them into the wilderness.
Then there are weird cases when the local Dark Pits or bandit camp "link themselves to you economically" which is representative of black market activities and other such not yet implemented goings on.
Nice! Getting these features added to the tutorial would definitely be a good idea, as managing populations has always been something you couldn't really do besides "keep up with the influx".
Being able to reduce migrant waves by sending them back definitely has some significant gameplay implications with respect to that, and probably should be one of those things that just about any player should add to their repertoire. I know the population issues have gotten to the point where players would sometimes construct in-site deathcamps for unwanted professions.
The FPS implications are pretty clear as well.
That said, being able to control population attraction in-universe, by making your fortress politically more difficult to join (being able to force migrants to use the petition process, being able to expel visitors (can we do that yet?), and so forth) would definitely be helpful as well, as managing who gets in by expelling one-by-one could get to be too much as well.