You will be fine with a Male GT under 8". Anything larger than that will need at least a 75 gallon. As far as smaller fish being housed with it, really depends on what the "smaller fish" are.
I would say go for it. Bout 10 inches for a GT is fair game. Get him as a juvie and if he starts to look cramp go bigger. You can easily easily find a 55 for 50 bucks, and you would already have the filter, and what not for the 46, so just change over. I think really anything over a 55 is overkill(unless you get some of them freak GT's that grow up to 12 in.). But to each his own.