Can SNEmulDS play famicom games too?
Or do I need another emulator?
etcetc
Famicon and NES/Super FC and SNES are practically the same (not sure if they are - AFAIK they are just different regions), so I don't see why it wouldn't work.
Each console is independent of each other and are not cross-compatible. In real life a Famicon could not play Super Famicon games and vice versa. What SpikeyNDS has said is correct. gab10 will need to use a different emulator for the Famicon games.
Oh and as for the region bit. If I recall correctly they put a plastic tab in the US released console in an attempt to prevent people from importing the japanese titles and using them in the us consoles. If you broke the tab then the us console would accept the japanese carts without issue at all.