Please don't get Discus as they need 10 gallons each with a minimum of 6. Ideally in anything beyond 60 gallons. 4 would work in a 50-55 gallon.
I think RTBS are like betta's - some are pleasant, others are horrors. The one my Dad has is a perfect tankmate, about 5 inches, but I think a lot has to do with the environment they're in to. His tank is heavily planted with lots of bogwood, and therefore feels secure.