Well, most reefers I know keep their tanks at 1.025, I personally keep mine closer to 1.026. I believe the natural sea water is closer to 1.027 SG so I'm sure if your close to that your fine. Now if your talking about a hypo SG tank for treatments of something like ich or something or that nature I would only suggest it in a separate tank...
For the inverts issue in your tank you can get any herbivore fish that will graze on most algae that will grow for example a lawnmower blenny or some tangs would do the trick...
Good luck...