Yeah, watch out for wildly varying wattages, and be careful about impedance.
That said, though...mixing and matching speakers sounds ****ing awesome. I have a 2x12 that has an Eminence Swamp Thang and an Eminence V12, and it definitely sounds much better/bigger/thicker/clearer/other buzz words as a result. I play metal; the Swamp Thang has a fantastic, prominent, cutting midrange and a very articulate sound, while the Legend has a sort of dry, almost sterile "modern" sound with a decent bit of low end, very smooth highs, and a kind of spongey response. Put them together, and you get all those qualities in one cab.
Do some research and experimenting; Eminence has a tone chart on its website that gives you a pretty good idea of what their different speakers sounds like. Just don't pay any attention to the "good for ____ genre" parts of the description. And don't worry too much about matching watts; you probably won't want to have, say, a 60-watt speaker paired with a 120-watt speaker, but I'd guess that a 75-watter together with a 100-watter would be fine.
Very old stage rig
Mourn not the dead
that in the cool earth lie...
But rather mourn the apathetic throng
the cowed and the meek
Who see the world's great anguish
and its wrong and dare not speak.
- Ralph Chaplin
"All good people are asleep and dreaming..."