You might want to first build a Champ/Princeton "stock" just to get your feet wet in construction. To answer your other questions, output tubes can be put in parallel - in fact different tubes can be put in parallel and biased so that, for example, a 6V6 conducts at lower signal levels and a 6L6 at higher levels. But let's stick with two 6L6s for simpliciity - if you have the amp cathode biased you have to have twice the resistance for one tube (or half for two tubes depending on how you look at it). Also your output tranny's primary impedance will have to be halved for two tubes - this can easily be accomplished by moving the speaker to different impedance taps on the secondary to reflect a different primary impedance - so you will need an OT with a tapped secondary - there might also be some issues with altering NFB resistors if you use it. A 12AX7 or such should easily drive two paralleled 6L6GCs in class A. Your 75 watt OT is probably made for PP and wouldn't be properly gapped for single ended operation - you'll need a tranny designed for SE operation.
And, hope I've hit most of it, even a single 6V6 can be awfully loud for "bedroom" use.