Depends on what you're stocking, how much you're feeding, other filtration, and what your maintenance schedule looks like. As long as you're doing weekly WC you're most likely okay without one.
No. I had a Innovative Marine skimmer on my Nuvo 20 and it didn't skim much and wasn't really stable. It was also super loud. The skimmer cup was tiny, so I'd have to empty it daily and it pulled nothing but lightly tea colored water. Not worth it, in my opinion.
I get really thick scum with the ghost protein skimmer. except now that I am dosing NoPox I am getting lightly colored skim.
to OP you dont really need it even with sps as long as you do your water changes