Take it from me: you can have several USB mice hooked up at the same time, and they all work. It is easier if they all come from the same vendor (like logitech), so you can just install one driver, and all their special features work.
Sometimes, even different brand items work with their drivers too. Right now, at this very moment, I have a Logitech MX510, an IBM glidepoint, and a keyboard with a trackpoint (eraser-head) pointer (also IBM), all working, all via USB.
On top of this, you can ALSO have ONE PS/2 mouse, AND one Serial-port mouse.
At home, I have a single USB, single PS/2, and single Serial pointing device. The Serial port device inherits the settings of the PS2 device, for some reason. And they all work.
Getting this all to work used to be a headache, but for the past few years, it has been a snap.
-Llama