In the MM, Rakshasas as characters have no bad stats (+2 STR, +4 DEX, +6 CON, +2 INT, +2 WIS and +6 WIS). SS has the starting abilitiy adjustments at +2 CON and +2 CHA. Given that these are its strongest abilities (according to MM), I wouldn't mind allowing just those two bonuses. Maybe add a -2...