For most things, he Magic Trackpad is amazing, but there a a few situations where you will want more control, in which case, any mouse or trackball will do. Logitech and Microsoft have some good, but cheap options.
Yeah... there's no reason you can't use both a trackpad and mouse simultaneously
Most of the time I use my trackpad on my Mini, but when I fire up Portal 2 I grab the logitech mouse from its charger, and certain apps that work better with a mouse (lots of clicking/dragging).
The only downside to the trackpad is that it eats batteries quickly. I've gotten into the habit of turning mine off every time I'm done using the system which really helps. I'm at about 6 months on my original set of batteries with about 60% life remaining.
My old logitech wireless keyboard (predates bluetooth) is super-efficient though. The original set of AA batteries had corroded by the time they needed to be replaced, and that's with nearly daily usage. When I retired my old Windows tower I moved that keyboard to my new Mini.