Mixing chain/cog sizes is no big deal (except that you can't run a 3/32" chain on 1/8" cogs and rings). I run a 1/8" cog and chain with a 3/32" ring and have zero problems with it.
A 126mm hub should be off-center to better accomodate the freewheel. You can usually just remove the spacer from the drive side and be at or much closer to 120mm. Even if not, stretching each set of stays by 3mm won't hurt anything, but will be a pain. Cold-setting the frame to 126mm is pretty easy to do if yo are careful about keeping things aligned and you don't mind doing it to your frame.