Say I make an HS mule...

Yes. But you need another leecher in that case. The cap is not 121%, it's 110%.

You cannot gain more than 110% EXP from a monster in that white text saying "You have gained Experience (3520)" or something along those lines. You need another text to get anymore EXP, one of those lines is "You have received EXP bonus for party (352)."

Ah, I understand. Lastly, does this 121% stack with 2x/4x etc.? Say, if I got 100 exp off a monster normally, and 200 with 2x, with HS would I get 221 exp or 242 exp?