that will work if you keep your bioload low. It's not the best quality skimmer but I did use one for years on my 54 and it worked ok. You could also do a search on modding it to increase its production. In terms of media in your HOB, I was wondering about carbon or other media (phosguard, GFO, ceramics). yes, the dusty stuff you see when you move the rocks is detritrus - if that settles, it can create excess nutrients - extra flow (will help keep it suspended so it can be taken out through your HOB and now skimmer). In terms of feeding, I usually feed my fish a variety of food (pellets, frozen) about 2-3 times a day. Since you have a shallow sand bed, I'd suggest vacuuming small sections (about 1/5 - 1/4) during water changes.
You'll also want to include a varied clean-up crew in your tank that will help with algae control. In terms of water changes, you may want to look at 10% of your water volume about every other week. You'll get differing opinions on that - some would say more percentage and/or more frequently (weekly).