Random, IMO, either species will take the same fly patterns. For whatever reason, there seems to be an attitude that flies for Smallmouths need to be smaller than those used for LM bass. I've not found that to be necessarily the case.
In some places that can be true, but the same could be said of LM bass in some places. If the average size of either is small, then the flies don't need to be large. However, in those waters where either grow to big sizes, the same flies & sizes can be used for either. For me, that generally means larger size flies. Either species of bass will eat the same things if those things are available to them.
There are some small streams I've caught SM's in, and yes, I used smaller size flies, even down to size 12's. These are streams where a 3 or 4 wt rod is a good choice. Tossing big flies with heavier rods in these streams is not practical
The bass in those streams don't get too big, simply because the environment doesn't allow them to get too big. There's the possibility that there could be a few larger size fish there, but the majority would be on the small size, so I'll fish flies of a size that best fits the stream & the fish. That does not apply to all Smallmouths.
I've caught some decent size (2-3 lb range) & smaller size Smallmouths in the Susquehanna River here in MD (below Conowingo Dam) on 8" long saltwater type lures while fishing for Striped Bass.
I've caught both SM & LM bass several times on the same big lures & some really big flies. IMO, either are not particular & will try to eat anything you throw at them.
I've never fished for Pike or Muskies but have read numerous accounts of some real bruiser SM's & smaller ones being caught on Muskie & Pike size lures & flies.
In the past, I thought flies for Smallmouths needed to be smaller, but I found out first hand it's a myth that Smallmouths won't strike at big lures or flies.
Whatever size flies you tie & use for either species of bass should fit the size of the fish, the size of the waters & of course the rod you're using to cast them.
Some fly patterns, by their design, are better for places with current & some are fine for stillwater or current situations, but frankly, bass are bass.