I am almost done earning my MMR but I did it in a variety of scales. My HO layout was primarily used for civil and electrical, though I scratchbuilt all my track requirements in G scale (switch, crossing, gauntlet) just for fun. My cars were done in four scales (HO, OO, On2, O) and my structures in two scales (N, HO), again just for fun. My author certificate was entirely in articles about 1/8 scale trains. But it has been a fun process!
All in three rail is quite an accomplishment.