For a system with multiple mice, there's a server layout bit lower down, from my system (one mouse) it goes:
Section "ServerLayout"
Identifier "layout1"
InputDevice "Keyboard1" "CoreKeyboard"
InputDevice "Mouse1" "CorePointer"
Screen "screen1"
EndSection
The mouse you want to use should be the CorePointer. If one of the mouse input device entries is redundant then it should be removed or commented out.
You may like to Google for this and related topics, or look on the X website,
http://www.xfree86.org/ It's quite possible to have many mice connected at once and use them in "mixed mode".....