...I would forego the sand...
You might consider that a protein skimmer would help with cleaning the water as well. Mangroves work well, but they can grow quickly and I really cant see how you could use them with this type of sump. I would stick with chaeto or some other type of macroalgae and use a plant bulb for lighting. A filter sock over the input to the sump would catch the majority of the solids from the tank. You just have to clean it occasionally.
You can keep a cleanup crew in with the live rock to help keep the detritus to a minimum. A couple of cleaner shrimp or crabs would be fine. I wouldnt put a starfish in the sump. You can, of course, assuming they can get enough to eat but I would rather see those in the DT.
When filling your sump I would advise that you only fill it to the level where the DT would siphon if you lost power. This way if I were to lose power I dont end up with soaked carpets.
Another option to add a 'fuge would be to change your sump to one like Kio shows in his pics.
Good advice. armyman has another post about his sump. It was manufacturer made and I think I read he didn't want to change his sump. The thread was very long though, so I stopped reading it. This was my thought after the first 2 pages on his other thread -
First chamber:
Get a filter sock and upgrade the skimmer (make sure it fits the first chamber)
Second chamber:
Throw away the bioballs and filter tray in order to grow macroalgae in the second chamber. Cut a square hole in the white cover top for lights to enter the chamber. No sand because its not needed or required. There is enough live rocks in the main tank for all the necessary biological filtration.
Third chamber:
Just a pump.
Nothing else is needed. More stuff can be added, but the cost of those items may outweigh the benefits.