I played Warhammer 40k for 4-5 years. Games Worshop required that at least 50% of the model be made up of Games Workshop pieces.
For X-mas I got a small Lego Lambda shuttle that is almost the exact scale of the FFG xwings miniatures. Sadly, it's top fin/wing is about 1/8 of an inch to high to fit into my battle foam.
The question though, is that assuming I can get the Lego one to stay on the base, could I fly it at official tournaments such as GENCON or the likes? Didn't see anything regarding this in the rules, but I probably missed it