Can I add Suntouch floor thermostat to 20 amp breaker circuit with in-line GFCI outlets
Go ahead

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).


and 210.23(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!)