Trackpad Issues - Should I let them prevent me from upgrading to Yosemite?
Your fears are unfounded, although kudos to you for being so conscientious about the unique circumstances of your machine before attempting to upgrade. In any case, consider an iMac or Mac Pro which must have an external mouse, there is no internal pointing device. You'll be fine as far as a USB pointing device goes as long as the mouse you're using did not require any special drivers when you first began using it.