I haven't kept matamatas, but I would be leary of keeping other turtles with an animal with such a large gape and big appetite; if the two animals go for the same food item at once, the softshell's head could end up in the matamata's mouth. Snappers often kill similar-sized tankmates this way.

That said, the Nashville Zoo keeps adult matamatas with various other similar-sized turtles; they must not have too many problems, or they wouldn't maintain that setup.