You should be fine. I run a basically a frag tank out of a 2 1/2 gallon with the exact same light, only just one of them, and even smaller filter, and all that really does is move the water arround. I have metalic green star polyp heaven going on in there, they must love it. If the rocks were smaller size I would have soldl/traded them by now, but as it is, I am waiting for them to grow in a bit more before I ask for the big $ for them - a bunch of four to six inch chunks of metalic green stars on purple mat. I have seen chunks like this retail for $40-50.
The star frags were placed up high in the tank BTW, so are only about 5 inches under the 18w fixture. That is why they grew down and over all my LR
Kenya does ok in there, as well. Not great, but not dead after 2 years either. It grows, just not fast.
Xenia same thing - small frag, 2 years later, is the exact same size. But it has been 2 years, so that must be saying something to keep this stuff.
My frogspawn frag isn't dead, but 2 years later is about the same size, and never really 'opens' up. I would avoid SPS
corals under these lights, as LPS
don't really do all that well.
All kinds of polyps and zoos do great however.
Good Idea about putting the rubble in there - that will make removing encrusing type corals a lot easier, mine liked to encrust over the LR
that was decoration instead.
Good luck and keep us posted on what works, I am looking forward to the day when I can sell off these star polyp colonies and try something else.