I think it's probably worth noting why NSF had different formats:
- NSF, the one that's standard no expansions stuff, used to be a catch-all for anything that could run on the NES. It was changed with the introduction of NSF+, which was intended for things to happen on more fair grounds.
- NSF Classic, I believe is meant as an analogue to FamiCompo's NSF Classic, which denotes a stock 2a03 with no DPCM, and no refresh-rate shenanigans (only once per NMI here, I believe regular NSF is different).
It's certainly true that it's got more formats, but it's also way more popular of a system and had several expansions for it. It's not "just because".
As for Game Gear, the only meaningful difference between it and Master System is the addition of stereo. If you count OPLL, then that's another difference - The Game Gear doesn't have access to that.
I could see the suggestion work on the grounds that the current format is renamed to Game Gear, and there's a new SMS format with OPLL allowed, but I honestly don't think it's worth adding yet another format (and potentially another badge) for what's a pretty insignificant change in reality. I'd vote to just keep things as they are really.
For tools, anything that can export Mega Drive could potentially also work for SMS. Specific tools include Furnace and SnevenTracker, the latter being a fork of 0.4.6 Vanilla FamiTracker to add both SMS and GG support.