I have a similar sump, ie small.
I baffled off a section in the middle as a refugium. It's basically... Drain+skimmer--->fuge--->return (with a bubble trap before return) Maybe 6" wide by the front to back depth of the sump...12"-ish for the "fuge", if you could call it that.
Put in some rock rubble along with some chaeto, red mangroves, green gracialia and red gracilia. My chaeto doesn't seem to want to grow, but the other stuff is going gangbusters. Mangroves are sprouting roots and I noticed last night the Red Grac might have doubled in size and is now sticking straight up. I think the other stuff is just starving out the chaeto. Or my tank is too young and bioload too low for it and the other stuff is eating the nutrients the chaeto wants. I do only have 2 fish at the moment and no corals.
Red and green gracilia are small, so those might work for you. Same goes for mangroves as seedlings at least. (Got it all from reefcleaners.org, poke around)
I use a 6500k CFL spiral as lighting. I don't think the heat is too bad, but i'm sure there are LED fixtures out there that could work as well. I know saltwatercritters has some nice clip on LED fixtures that might work well for you.
My nitrates where over 60, maybe pushing 100 before my macro installation. Been 0 since. Again, low bioload still, but it makes me feel good that when I add things, the macro is established enough now to catch up when more nitrate producing activities are happening. I think it's effective because given my lay out, all the water has to pass through the fuge before it hits the return rather than relying on some % of it being pumped into an external fuge and then returned.
