Red Tail sharks aren't as agressive as people make them out to be. I have had them in large tanks (in my 6" I even had 2) and also in my small tank (Juwel 96). They like to play Tag, so they would probably prefer fish which don't mind that game (YoYo loaches, Danios, Tetras), but they never harmed a fish or eaten any (not even Guppy Babies, sigh - one of the reasons at the time to add him to the Juwel). I always have heavily planted tanks and generally lots of caves, which probably helped. They are also very hardy and graze algae (bit like a fancy false SAE, but never as good as a real SAE).
Although they love bloodworm (which fish doesn't..), I think they may be mainly a herbivore so be careful if you feed a lot of bloodworm. They are very long lived, the first one we had lived to 10 years but then he got killed during a sudden and very unexpected frost (we used to put our tropicals in the garden pond during the summer in the old house). He was the sadest loss we had, as after all this time we became very attached to Mr Sharky. The 2 in the 6" went back to the LFS during my last move, as the tank has been dry since - just haven't got the time anymore, but were 5 years old then. In the 6ft tank they had plenty of room to avoid each other, but I was surprised how little they squabbled. Apart from the old game of I chase you if you chase me once in a while, they ignored each other. If anything they seemed to be glad of each other's company, as none of the other communals enjoyed "tag" and just swam away, leaving the shark quite bored.
I wouldn't add him to a tank which is low in stock, as his chase drive can stress other fish out, but shoaling fish are good as are lots of other small fish which aren't too agressive.
