Depends on the size of your tank, but you are right, they do better in 8 or more. 6 is a minimum IMO.
As for other tankmates, I've kept them with Plecs, Clown Loaches, Sajica Cichlids, Platties, and Tetras (neon and black neon), but as long as you avoind long finned, slow moving fish like you said, you should be OK. It is a nit of trial an error really, I've read some people saying that even in a group of 8 they attacked and killed anything that went in the tank, but mine never bothered any of the others, they were too busy playing with each other!!!