personally i'd do a 30 or better yet like a 55 or smoething. the 30 will be cheaper but the 55 will be easier to care for. a 20H is fine but make sure there's somekind of aeration, a sump for a filter or lots of pwerheads aimed up or a HOB filter or something, cause marine tanks don't hold as much O2 and 20H's don't have a lot of water-air contact area.
some good fish would be firefish, clowns, damsels, gobies, blennies, if you go to liveaquaria.com there's a whole section for nano SW fish.