I have 6 dwarf rainbow fish and they are just awesome; one of my favourites for sure. We had 5, but were male heavy, so we added another female to balance them out and now they are quite happy together. The 5 had the males squaring off and almost fighting each other. The
LFS said we'd probably have to up the female ratio to get peace, and sure enough... they were right.
They never bother any of the other fishes in our tank and really add to our community. I'd highly recommend them.
They get to 2 inches (6 cm) and I read somewhere that they can be jumpers, so make sure you're tank is secure. As well, they need a well established tank to join up as I also read that they can be sensitive to poor water quality/overstocking. They also enjoy a bigger tank - min. 20 gal.
We have found them very easy to keep and ours tend to swim at the top to mid of the tank and are incredible to watch the shoaling behaviour.