Any fish with 'barb' in its name can cause issues in a tank. I'd go for a group of at least six dwarfs. Rainbows are stunning fish - even dwarf rainbows, once in your home tank will develop colouring far above what you see in a shop situation. Remember to give any tank with rainbows in some open swimming spaces as they do love to zoom along the tank. They will happily adjust to the pH of your tank. I had boesmani, reds, blues, dwarfs etc and they were fine in lower pH. If you do find you have a pair of them and want them to spawn then would be the time to raise the pH to the optimum level.