What I need to know, from the people who have done this before, is how to do it? What issues do I have to fix? What problems am I likely to face? How do I move over only the trackball assembly and nothing else?
Will do a better write up later (if not ping me) but here's some basics:
If you open the trackball you can remove just the controller and cut eff the buttons / housing. Then put this in your KB and run a second USB out for the trackball
Things to worry about:
1. Space is a huge issue when working with trackballs. You will likely have to either cut out a huge hole or put the rest of the KB up on risers or at an angle
2. mounting the trackball assembly. Once it's liberated from it's shell you have to figure out how to attach it to the KB at the proper height. Superglue is a good solution for some.
3. If you wait a little, you can have a much better trackball using those laser sensors mr. Kicklighter made and some ball transfer units, To cheaply mount the ball transfer units you can just get a rod of something, cut the end off at an angle and glue it in place (times three) and mount the sensor under it on shims until it's close enough to work. Controller can go anywhere.
It is possible to just move over the trackball assembly and not the controller, but you'll have to do a lot more work to save that space. The buttons are likely either on PCBs attached to the controller via ribbon cables or small switches on the controller PCB. If they are on wires, detach or cut them. If they are on the PCB just leave them there (they don't take up much room)
If you open the trackball and the KB it sholud become clear how easy / hard it'll be to achieve fitment, as well as how many keys to remove. A good place to start is to have a hole the diameter of the trackball in the KB