I have a dedicated 20 amp breaker that feeds a dedicated circuit for all the GFCI outlets in the bathroom (5). I also have a SunTouch floor thermostat attached to a warming pad under the floor that says it can go on a 15 amp or 20 amp circuit depending on the size of the floor. I was planning to attach the thermostat in line (series) with this 20 amp circuit. Does anyone foresee any issues?
The Suntouch has a built-in GFI. Don't connect load sides of GFI's to other GFI's. They'll burn each other out.
If you're running a 120v mat, you're going to pull alot of amperage. Best to see if you can try a temporary hookup to the circuit in question, then try a hair dryer (and/or whatever other appliances you'd run on that circuit) on that circuit at the same time the floor heat is on, to see if it'll hold. You'll probably be pulling somewhere near 11-15 amps (that's a guess, you need to check) with that size mat, so you're getting close.
You can attach the mat and thermostat to this circuit -- a branch circuit feeding receptacles in a single bathroom can feed any other outlet in that bathroom as per the Exception to 210.11(C)(3):
(3) Bathroom Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one 120-volt, 20-ampere branch circuit shall be provided to supply a bathroom receptacle outlet(s). Such circuits shall have no other outlets.
Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).
(2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.
If it provides its own GFCI, you can connect it to the circuit coming in. Otherwise, you'll need to provide GFCI protection pursuant to 424.22(G):
(G) Ground-Fault Circuit-Interrupter Protection. Ground-fault circuit-interrupter protection for personnel shall be provided for cables installed in electrically heated floors of bathrooms, kitchens, and in hydromassage bathtub locations.
Last but not least, the thermostat should control the heating mat and only the heating mat. Not only is it probably not rated for the entire load of the branch circuit, it'd be downright daft to thermostatically control the receptacles in the bathroom. (Good luck with your hairdryer in that case!)