I have a question on YZE OGL: the engine is the base system, but can a specific rule by another game be used under the OGL?
While I'm neither an employee of Fria Ligan nor any sort of copyright expert, I think
it's permissible to reproduce rules in your own words
under normal copyright, at least in the U.S.. (Copying text from copyrighted rules, on the other hand, is plagiarism and a big no no.)
If a word, phrase, or block of text is listed as "Product Identity", then under the OGL you can't use it ... but Product Identity is usually listed at the beginning of the document (EDIT: or the license at the end), and all I see in the YZE document is boiler plate for someone else to insert their own Product Identity. For safety's sake you might want to avoid specific terms from other games, e.g. "Dark Between The Stars"; call it something else. "Action Points" is a general term used in unrelated RPGs, so it might pass muster.