You can always try to study the MU and be on point.
7-3 is by no means an unwinnable MU, just means that you'll have to work harder.
You can always try picking a sub character for those characters, but what happens if you are playing a sub character against a character you really don't know the MU off? It's always better to stick with your guns, enough people have tried picking up subs for certain characters but they always go back to their main(s). Biggest example is Tom Brady.
I'm suggesting that you don't pick up a new character just for 2 MU's.
I don't think it's necessary and
XBlades will agree I think