Riftborn have nice bonuses--three out of four FDSI. I used culture to flip some systems from them, but their numbers don't grow in the empire.

Looking at the pop screen, they are distinct from my other races in that there's no luxury resource to boost their growth. And of course, they don't eat food, and I don't have their ability to construct more from the build queue. 

So, is there a way to build up what I have? Maybe a faction quest? 

I think the only way is either invading more systems with riftborn or waiting for them to migrate from closest AI systems. You can know they are migrating if one of your systems (usually the nearest to AI's) has a little 'human and an arrow' icon in the system view.

Oh, I didn't know they migrated of their accord. Interesting.

While I'm at it, I think I'll throw in the reverse issue. I also absorbed a Horatio system. The pops are growing too quickly, given that they basically do nothing for my empire. I don't need the food, don't need religious pops. They're taking up shelf space better aliens could be using.

Now, I know I could just pack them into shuttles and send to planets with no room for them, and they'd die like bugs on a windshield. But, y'know, it doesn't seem like a legit for a benevolent dictator to solve population issues. I don't have some way to send them back to Horatio, as far as I can tell. I would try just packing them all on a system that's full, but it seems like that doesn't work with my race, so I suspect these guys would just overflow to other systems as well. 

You can reliably use chain gang programm (tier 1 military tech) to turn them all into manpower or a dust virtualizer (i think its called that..) to turn them into dust. However the last one can be only aquired from curiosities. Horatio are kinda nice for Super Biofuel Factory strats though with their +3(6) food/pop and +15% food from 20 pop collection.

Yeah, food just doesn't seem to be a problem for me. If amything it's causing my planets to fill up so fast that I can't manipulate them for purposing of growing alien races. 

So, thanks for the info on Chain Gangs!