when making familiars or spirit creatures, especially with the advantage can they take advantages/disadvantages. and if yes is there a limit on which they can take?

I think, they are all created with the rules for "Creation of supernatural beings" from the core book. Therefore, they can't take advantages/disadvantages with CP, but can buy them with DP (as "Essential Monster Abilities" or so).

With Sheelas I'm not sure, because I don't own the ArcanaExxet.

So long,

Sheeles themselves have predetermined stats which cannot be changed unless when its master levels up (In case of familiar). Each level the Sheele may add +1 to any attribute of the masters choice and choose a power which is described in Arcana Exxet.

As far as i interpet the rules, the base level of the sheele is up to the GM but it should follow the basic rules of familiars.

The base level of a Sheele is determined by the level of it's master, it being equal. They aren't really created separately from being a bound part of someone, as they are a physical manifestation of their master's soul. For other creatures, like has already been pointed out, some Advantages and Disadvantages show up on the list for the Creature Creation section in the base book, and Those Who Walk Among Us has additional ones available, and their prices listed. CP's are basically PC gifts, and also apply to select other NPC's. Theoretically, they could apply to all humans (and Nephilim), but I'm not going through and giving all my Commoners CP's.