I have kept and bred them, don't have them at the moment though. They will not bother, or really be bothered by Platy's. How much you can keep depends on the size of the tank and what else is in there.
They are quite small as Rainbows go, reaching about 50mm, females are a bit smaller. They were easy to keep and feed, although they tend to take finer food then other similaly sized fish. They did not seem interested in sunken food however. They liked live mosquito larvae.
The males display to the females, (and in my experience, most other small fish!), by flicking the anterior dorsal fin up and down - quite comical to watch. They spawned readily enough on floating mops. The fry are small and difficult to feed. I frequently lost a female after spawning - I ended up with a lot of males. Over a couple of years, I guess I raised about 50 to "swappable size".
As a rule, they kept to the upper part of the tank but exclusively. They would happily swim in open water in a large tank, but I kept them in with very docile quiet fish, Rasboras and Popondettas mostly.
Quirky side note, I won the best in show at Mid-Sussex Aquarist Society's open show with one of these about 20 years ago, although I guess, they were very much rarer in the trade back then so it had novelty value.
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