Conduit Fill and Derating for THHN Cables (outdoor run)
I highly recommend using 20A for both circuits

Typically 2 bathroom circuits means lights + receptacles:


Lights - This can also be exhaust fan, or heater fan. Lights and exhaust fan don't need much power, but a heater fan does. So even if you don't plan on a heater fan now, I suggest planning for the future by running 20A instead of 15A.
Receptacles - This should be 20A to allow for usage of multiple devices. One 20A circuit won't allow for "anything" (e.g., 2 x full 15A devices (which would actually be 12A continuous = 24A for 2) at the same time), but it will give a much lower chance of nuisance trips than 15A.



  Since I will be converting the THHN/THWN-2 cable to Romex 12/2 or 14/2, is it okay to join the green sheathed grounding cable of the former type with the bare copper one from the romex?


Yes, that is fine. All grounds together - doesn't matter green or bare.

Also remember that the connections must be in an accessible junction box.


  If I use 12 gauge THHN on both circuits, am I limited to using only 12 gauge romex inside the house? This is what I am assuming, but just want to make sure.


The limit is based on the breaker. If you use a 20A breaker, then all wire must be 12 (or larger). If you use a 15A breaker, then you can use 12 and/or 14 (or larger, but why?), but a mix could confuse someone working on it in the future - i.e., if they see 12 connected to a 15A breaker then they might think it can be upgraded to 20A, when it can't because part of the circuit is only 14.


  Is there a benefit to using stranded vs solid THHN?


Yes, easier to pull. Probably not much of an issue for what appears to be a pretty short run.