I owned 8 dwarf neon rainbows(all succumb to columnaris). They lasted 5 months with me and they have spawned prolifically in my tank. Too bad...In the community, my other fish have free meal on eggs. Some eggs did hatched but the fry were eventually devoured.
IME, they are quite prolific. They are good community fish and grow to 6 cm max. They are also easy to keep as long as you maintain the tank well otherwise they'll succumb easily to columnaris.
As these rainbows prefer soft water, pls choose their tankmates carefully. I suggest you keep them with fish from Amazon origin.
As Eddie said, males have red fins and females have yellow/orange fins.
I'd keep those rainbows in a tank of black gravel and pink lighting. Their colors go out excellently.
Feed them with small chunks of food. They may have large mouths but their throats are quite narrow hence my advice to feed them only small bits of foods.
Incidentally, I also have threadfin rainbows(Iriatherina werneri
) but their fins make them victims to fin nippers. Threadfins are best kept by themselves but I have kept mine in a community successfully although it's not advisable. As long as you know how to choose the tankmates for the threadfins, you'll be fine.