Yes, for simplicity assume no overcasting by creatures, unless you want to fully develop some member of the "races" as an NPC, in which case follow normal rules. I would assume 1 rank per level for most creatures, although 2 ranks/level may be appropriate for some extremely magically-oriented creatures or for those whose power is innate rather than spell-cast.
It is unclear to you because the rules are pretty incomplete when it comes to creatures. There's also no guidelines for creature stealth (is it harder to detect a tiger or a Greater Spider stalking you?) or a lot of other areas. Instead, RMSS/FRP throws in redundant stat blocks and bland flavor text to pad its enormous C&M page count. The coming revision of Rolemaster may not give an immediate answer to every question about a creature, but it will provide tools to let you figure it out in a sensible way rather than winging it all.