What happens if I parallel two somewhat different size transformer primaries?
The Japanese (back when 50/60Hz mains transformers were still viable) would often use a lighter gauge of wire for the 'upper' half of the 230V (or whatever) winding. North American makers, in my experience, not so much. 

I don't think bifilar winding would be good because of insulation considerations when they are in series. It's fine when they're in parallel (and sometimes used because of skin effect or just to get a heavier gauge), but I think bad practice to depend on the wire insulation for much breakdown voltage. If you break the input to the transformer with a switch, a lot of voltage will appear across the primary winding, and half of that would be wire-to-wire with a bifilar winding.  

You can't connect them in parallel because of the CT, but it wouldn't be a very good idea anyhow- the different number of turns means the transformer would quickly overheat.