It depends too if you are getting babies or older fish. I keep 6 babies comfortably in a 55, and know a successful discus breeder who does the same . . .more imperative than tank size from my experience is water quality. I keep them with cardinal tetras, small clown loaches, and a fire eel and I've never had any problems. I keep temp at about 82, which is good most of the time but you might want to keep the option of bumping the temp a little higher if the Discus get sick and keep that in mind when selecting tank mates. Corys are generally a good choice too. I'm also not sure about the sword tails, but I wouldn't keep fish with them that are fast moving and darty swimmers (like Bala Sharks) even if they are peaceful as the constant movement with stress the Discus. Don't overdo it with the tankmates though, the Discus need to be the focus of the tank. Hope that helps . . .