How big is the tank, and how big will the permanent tank be?
My male eastern mud bit the tail off of the female eastern mud. She recovered fine, but I learned my lesson. At a nature center I noticed a painted turtle being kept with an eastern mud in a 30 long - the painted turtle was missing part of it's tail, too, and had some wounds on it's hind feet. Yet another nature center has a pair of eastern muds in a very large indoor pond with other aquatics, but there are also many hiding places, and the muds stick to those while the larger turtles ignore that area - they have no problems there. If you have a VERY large enclosure, and hiding spaces seperate from the basking area (so no body parts are passing directly over a mud turtle hiding area), then it might be OK. Otherwise, I wouldn't mix.
FYI, male painteds can be VERY aggressive - no one ever told me this, I'm learning as I go in rescue, and I've getting in more and more calls about male painteds beating up on other turtles. I've gotten in a few other turtles that were kept in cramped conditions with male painteds, and they all had wounds of some sort. I don't see this problem with male sliders, what about others here?
Katrina