There are rainbowfish that range in size from 1 inch (Pseudomugil gertrudae) to 5 inches (Glossolepis incisus). Most rainbowfish are between 2-4 inches long.
All rainbowfish should be kept in groups of 6 or more. 8-10 rainbows make a happier group than 6
Iriatherina werneri and Pseudomugil gertrudae will be fine in soft water, as will most of the smaller Melanotaenia species like M. papuae, sexlineata, pygmaea, nigrans & exquisita. Many of the bigger species will be fine too but if you want small fish like tetras, then avoid the bigger rainbows.
Apistogramma dwarf cichlids should be kept in prs (1 male & 1 female) and do well in soft water. Most of them will also breed and it is interesting watching the parental care.
Apistogramma cacatuoides is my favourite and one of the most colourful, they are easy to sex, easy to breed, peaceful and live in most conditions.