Only keep one species per tank if you want to breed them because they will cross breed with each other and produce unwanted hybrids. However, if you don't want to breed them, you can mix different species in the same tank.
Make sure they have plenty of vegetable/ plant matter in their diet.
Keep the gravel and filters clean and do regular water changes.
Make sure any new water is free of chlorine and don't over dose with medications. Rainbows are sensitive to chemicals.
Try to keep the PH close to neutral (7.0) or slightly alkaline (7.0-7.8). In acid water or water with a lot of nutrients (high nitrates) the fish will become nervous and skittish and can die from shock.
New rainbows often damage the tips of their nose/mouth. It will get a little white bump on it but it is not a problem and will heal up by itself.
Most rainbowfish problems are caused by poor water quality, dirty filters & lack of plant matter in their diet.