And regarding the Elven Skeleton

How can i tell the chance it will be up at any given time? How does spawn chance (in spawn group entry table) work? Is it only used for chance between multiple npcs in the same group, or can it be used to make an npc not spawn so often?
I suppose i can put a bee and a wolf in the same spawn group for 33/33/34% to reduce the chance of the elven skeleton?