So I have never really been into slap and I've been kind of lax in theory, just took what I got from a years lessons and went with it. Anyways, I'm ready to change that.

Right now my only bass that's playable is a Mustang with some roto medium scale flats. I know I suck at slap, but this just sounds like nasty... Do I need rounds and probably a 34" for slap, which I'd be using for flavor not flea antics? If not whats some tips?
flats and slap will never work together. ever. im surprised you haven't ripped all the skin off your fingers yet haha.
Short scale slapping is perfectly possible, see here. However slapping flats won't sound good at all - if you really want to slap on that particular bass I would suggest buying roundwound strings for it.
Professional lurker since 2009.
Switch out to some rounds, make sure your bass is set up properly, and practice your ass off.

If you think your slap sounds nasty, technique is the first thing I'd recommend working on. Good slap technique is hard to get down, and good old fashioned practice will help it sound better. Bad slap sounds nasty, no matter what bass or strings you're using.
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Spector and Markbass
Oh no, not that bass in general, that's just all I have, and I wanted to know if there was a remedy that didn't cost money. But, I'll probably buy a bronco soon anyways and puts rounds on it.