I have personally seen FWSM Running 3.1 and 3.2 with both 12.1E Train on the SUP2 and other trains on the SUP720. But, here is something that you may want to definitely consider. In case if you run into any issue during the installation or upgrade and call TAC for reactive support, the first thing you will be informed is, you are running a version on the SUP720 that is not compatible.
If it helps you make a decision, 12.2(18)SXF Train has been widely deployed in many environments and has been pretty stable on the SUP720 :-)
Regards,
Arul
** Please rate if it helps **