Since I am pretty sure they mix in the wild I would say yes, as long as they are of equal size, a size difference will work in the favor of the bigger turtle, eating more food
I agree. As far as I am aware you can mix the two species. However, both get quite big (if yours aren't already) and you'll need quite a large enclosure. Approximately 10 gallons per inche your turtle is x2!!!
they are both fairly aggressive i've kept easterns and yellow billies together the sliders both get bigger if they are females the males of the sliders are close to the size of the female painters.