I tried a Logitech trackball when they first came out as I was starting to get RSI on the wrist. But it soon gave me RSI of the thumb. So I went back to the mouse and made myself ambidextrous. I split the usage to both hands.
When I google for a mouse+trackball, they look like a mouse with trackball. But I can not be sure if they work as a mouse too.
Looking for Mouse / Trackball combo, not much larger than a normal mouse size.