2000 Protege ES, 85k miles, Front brakes

I'm going to replace the pads on this ES and my question is... should I replace the mounting support for brake pad? I'm concerned that corrosion on the support will cause a tight fit (if not impossible) of the new guide plate and brake pad.

Not sure which part you mean by mounting support, I think you mean the fixed part of the mount that the pads set in. I just put a set of pads, rotors and new caliper pins on my daughters car.
There should be no reason to replace any kind of mount, unless it broken naturally. Give it a rub with a wire brush and a file and you're good to go, watch you don't loose or break the s/s guides or the rattle springs.....Mike