I'm looking to start building a 2 x 12 cabinet once I have all the materials and whatnot readily available. Currently, I've looked at a Celestion G12T-100 and a Celestion G12M Greenback rated at 25w.

I'm currently using a 100W head, and was wondering if buying a pair of the G12T's would be a bad choice since the speakers really wouldn't be driven much.

- Need help deciding on a speaker configuration for a 2x12"
-Are two 100-watt speakers too much for a 100W amp?
-What would you recommend? I play primarily metal and need a nice low end. My other cab currently has Vintage 30s so I don't need another cab with them.
I'm doing a cab now with a Eminence The Wizard and a Governor.

The cab will have the speakers in a "slash" disposition, like this:
| O|
|O |

The Governor will be at the bottom.

Don't know if it's going to be any good yet, but I surely hope so. It'll be ready by next Tuesday, so I'll let you know.

A 100W amp can output more than 100W during peaks. Add to that the fact that distorted signals also produce more heating in the voice coil than pure sinewaves.
So your speakers need to be able to handle more than the 100W the amp is rated to.
150 - 200W total power handling should be used for safety.
Speakers that have the same impedance will split the power equally between them whether they are in series or parallel. So you'd want a minimum of 2 x 75W speakers (equal impedance) for the cab. 2 X 100W would be better and more common as well.
Moving on.....