FWIW I have 8 adult Multies (well, some are still small) and a handful of fry in a 10g. I have probably 7 dozen shells though (seriously)...and the bottom of the tank is covered in them. I haven't had any issues in the 3-4 months that I've had them, but I have plans for a larger tank in the near-ish future.

I am starting to think that I only have 1 male though, but alot of them aren't fully grown, so I could be proved wrong.
A 10g would be ok for short term with a small group. I only had a m/f pair for a couple months before I added more and got more than I really wanted for this size tank.
I know they'll soon fill up a 10g so 20 long is definitely a better size!