Muay thai vs Boxing
tough call, depends what u like basically,
if u just want to supplement groundwork i would say boxing, because, no matter wat anyone says, boxers have better hands than muay thai guys and u dont want to over complicate ur game.
However knees, elbows and the ability to at least defend kicks are all a necessary part of mma training. the other thing is muay thai does clinch work so if u prefer clinch take-downs (as apposed to shooting) then muay thai may fit your game easier.
like i said depends wat u like, maybe do both and see for yourself