@mrmarblz The trackball will need an interface board. Some arcade trackballs are sold without that, so be sure you have a solution for it or add on the interface if you have that option. The controller board will accept the wires from the trackball and will connect to the Pi via USB and should appear as a mouse to the Pi. Same with each light gun. If your table is wired with AC power your USB hubs can be powered and plugged into AC inside the table, so you can add as many hubs as you need.
If the Servostik is just a USB device, it gets 5 volts that way. The current draw would be as much as any gamepad plus maybe more while the server is engaged. Why would this be a problem
I would definitely use a powered USB hub for all of these devices, as you don't want the power to come from the Pi's AC/DC adapter. You don't want the Pi to have power issues because of all of the peripherals attached, and powered hub(s) should take care of that.
In order for the removable arcade controls to connect via USB, they will need to be wired internally to some kind of interface. There may be good choices for this, but a cheap option might be to hack four gamepad controllers and just wire your controls to the buttons etc.
This is an ambitious project. Check out my 4 player cocktail system. Mine is a bit simpler, but I had different challenges. My only question for you is: Why, no spinners?