Livebearers should be fine. On the rainbows, it depends whether you have both genders. If you have a male and a female, they almost certainly will breed, and then things will get a little bit tense for the other rainbows in the tank. I don't think anybody will get hurt, if there are territorial breaks in the form of driftwood and plants, but the nonbreeding rainbows will find themselves confined to a small area.