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I dont see why he wouldn't be fine in a 10gal. I have a dalmation lyretail molly in my 10 gal with my guppies and he's perfectly happy. From what I've seen in my local pet stores all the silver mollies are smaller than the lyretails. That's just an observation of mine. Not a rule at all.
A 20L or larger is best for mollies, they can get rather large. Some people suggest even larger, 29 or bigger. Obviously more water volume is better, but I think the 20Long footprint affords a little bit larger fish than a 20H would.