#1
I've been thinking about this for weeks now and its really starting to bug me. I know exactly what I want as far the cab goes except the speakers i want are a g12m greenback and a g12-k100. The problem is the greenback is rated at 25w and the k100 100w, so wouldn't the k100 overpower the greenback?
#2
not necessarily. The power rating doesn't represent the sound output which is more to do with the efficiency of the speaker at turning electricity into sound. Mixing the two speakers will give you an overall power rating of twice the lower rated speaker so you'll have a 50W combination if you use these two. Look at the figures for sensitivity which will tell you how loud these speakers will be with one watt. They'll be something like 97dB/W. If the figures are within 3dB you'll probably get away with them together, more than this and one speaker will definitely drown out the other.

If you want more try this http://www.ultimate-guitar.com/columns/gear_maintenance/making_it_loud.html