There is about 250MB available out of the gate. Really 300 but saves, the UI and such take up some of that
What is RetroArch? RetroArch is an emulator frontend that allows for multiple emulators (cores) to be used for a variety of consoles. Previously, RetroArch was successfully added to the NES Classic, which allowed for expanded compatibility of non-working NES games as well as games from other consoles including Atari, Sega Genesis, Game Boy Advance, and N64, just to name a few. The SNES Classic can also implement RetroArch for the same purpose.
I don't know how extensive they plan to make it. You can always just put it in and see if it works or not. If it doesn't just plug the SNES back into hakchi and remove it
If you were to have one save state per game (as example), on average, how many games would you recommend could be fit onto the SNES classic on average (i am doing this tomorrow, so want to finalize things)