If you want the skinny on current MLB playoff roster rules, may I suggest you check out
this thread? Forum member Le Grande Orange has done some excellent research to uncover the current rule setup, which MLB unhelpfully does not publish in full.
Unfortunately OOTP doesn't adhere to these rules (which I suppose is understandable given that a.) they're largely unpublished, and b.) they're constantly changing). You would have to take control of AI teams and manually prevent them from making moves in violation of these rules if you want them enforced.