AST is only in there till he is big enough to safely live in his own tank, he is too small right now to go into the other 90 Gal I have for him, and to be honest he is the most docile of the turtles. The Sliders all go into a pond next summer, and the Softshell will have the tank all to himself.
I've keep AST's before in large enclosure by their-selves, and when young they don't fare well cause it harder for them to find food since they are rather sedentary for a turtle. And I think a lot of people think AST's are like CST and really aggressive which isn't the case, I have kept adult AST's with other Turtles near their size and not one issue. Something that cannot be side of CST's at all, who tend to attack anything that moves. I think it comes down to the fact that AST's are by rule, a more specialized feeder than CSTs.