Guppies tend to eat their own babies. Some may survive, but not many. Most fish will eat fry in the confines of a tank.
If you want to prevent runaway breeding, thin out any dense cover in the tank. You'd want enough cover for your adult fish to be comfortable, particularly plecos like some cover over their heads, but not thick grass or tiny spaces that fry can get into without being chased by adult fish. No place to hide, and they're easy prey.
Alternately, you can stick to just males for any livebearers to prevent births (Females can still continue to give birth in the absence of males if they mated before you got them). Male livebearers are easy to identify: the anal fin is formed into a tube held close to the body.