For an old RM2 player I GMd, whom was a Bashkar/Runemaster/Seer split, we had fun with it.
I especially remember the many bar fights I would love my PCs getting into where the Bashkar(Barbarian Berzerker type) would be mid-Martial Arts swing and change into his non-MA Runemaster!
Oh, the hilarity.
Don't know RMFRP stuff, but we used a skill called Control Lycanthropy(basically a Self Discipline check) as a coping mechanism to help resist changes during "stressful" situations.
Depending upon (100-SD Temp stat)/10 for each individual personality's Mental Stats, this was how long their conscious states would survive for - measured in hours(even days for doozie triggers!) before reverting back to the main consciousness(or requiring a check to do so(could be an option for a more everlasting change if failures add up).
These included combat/gymnastic/social interactions where a skill rank wasn't at 10+(Or higher) in a skill that would normally be used in such situations.
So the more skilled the main personality became(levelling up), the less frequently the episodes of new personalities came up.
Sometimes, I'd be evil and add modifiers for moon(madness) phases of +/- 10 to the SD roll, per phase, too.
The range went from -20(Full Moon) to +20(New Moon) but these are tweakable, too.
We played in Shadow World, so moon phases were simple and the Kulthean calendar was a huge help.
I won't comment on the Demon/Guardian thing so you can milk our GM for as much as you can get, whichever way it goes, heehee!
Good luck, either way, sir!