RMSS and RMFRP are mathematically the same although RMFRP has an additional bonus.
In RMSS, you roll 1d100OE + 50 + your skill bonus in the list (basically the number of ranks) + any mods. Full success is at 111.
In RMFRP, you roll 1d100OE + your skill bonus in the list (same as above) + any mods. Full success is at 61.
So the difference is just that RMFRP moves that +50 from a modifier and instead bakes it into the table. The skill bonus is each case is your list ranks (but diminishing returns above 20 ranks) + realm stat + list type + other mods.
In RMU, you roll 1d100OE + your ranks in the list + realm stat + list type + any mods. Full success is at 1.
So, effectively, the bonus you are adding is basically the same across all the systems.
Personally, I read the RMSS rule "the only other allowable actions are movement and maneuvers" as no spellcasting, but using the term "maneuver" here is a bit ambiguous since everything is technically a "maneuver" in RM, even a "spell-casting static maneuver." In RMU, you just take a penalty from being stunned which is pretty straightforward.