Maximum Supply Voltage Output by Transformer - Sanity Check
Is AC in general by-default expressed in RMS values?
  


Yes. AC voltages are usually in RMS. So when we say the wall outlet is 120VAC, it means 120Vrms, which is 170Vpk or 340Vpk-pk.

The reason is because it's more straightforward to do power calculations with RMS. If I tell you the wall voltage is 170Vpk, you have to jump through some extra hoops in power calculations. AC current is in RMS for the same reason.


  
    Is the "120VAC" here in the US actually closer to 170V peak-to-peak? 
  


Not peak-to-peak. Just peak, as in zero to either the positive peak or negative peak.


  
    I was under the impression that all AC expressions where peak-to-peak, and that if a transformer was outputting 25VAC on its secondary, then a full bridge rectifier (and capacitors for smoothing) was just going to output something close to 25V DC. Is that not correct?
  


Not correct. See above. Of course, if a transformer outputs 12Vrms for 120Vrms input, then it will output 1.2Vpk for 12Vpk input. Since the ratios factors are linear.