How long, wide and high is the tank?
What sort of rainbowfish were you looking at? Most adult rainbowfish will eat neon tetras. Smaller species like Melanotaenie praecox, pygmaea, sexlineata, maccullochi and Rhadinocentrus ornatus and the Pseudomugils should be fine.
Neon tetras & Danios need to be kept in groups of 10 or more.
Guppies and Platies should be kept as either males or females, or if you want a breeding group, have 1 male and 5 or more females.
Rainbowfish need to be kept in groups of 8 or more but you can mix species of similar size and they all hang out together.