Firstly, welcome to the forum, and the hobby!

Secondly, people are right when they say six buttons for players 3 and 4 are overkill. As far as I know, no 4 player game needs more than two buttons for those players. But, nothing stopping you from going six buttons if that's what you want. It just means a (slightly) bigger CP.
Thirdly, the most common admin functions are: select game, exit game/ system, coin insert(1-4), player start (1-4) and pause. Alot of people use a shifter button to prevent accidental exiting from a game, for instance. The shifter can be your pause button, or any other button, for that matter.
As to trackball buttons for mouse, I'm afraid I can't advise on that. But I'm sure someone else will
