#1
I've got a 120w Peavey XXX 2x12 combo.

If I removed 2 of the power valves and one of the speakers.. would that work like a normal 60w 1x12 to reduce weight? I could even get a new baffle to fit the remaining speaker in the middle of the amp.
#2
Make sure you remove the tubes in pairs. So you'd remove the two outside tubes or two inside tubes. Also, this can throw your bias off. If you know how, adjust your bias for the two tubes after you've pulled them.

I don't know how the speakers are wired in a XXX 2x12. If they are parallel then you'll be fine to just pull one speaker. If they are in series then the output transformer impedance switch will need be be a lower impedance setting.