I have a clam and an anemone under t5s. My tank is 24 inches deep (3 inches of sand under the critters places them 21 inches beneath the water surface). I have a tek fixture, 4 55w lights with individual reflectors, and the lights are overdriven with an Icecap 660 ballast. I also have a separate 1 bulb 39w T5 fixture I use for dawn/dusk effect.
IME clams & anemones do fine under T5s.
I agree with Justin - individual reflectors on T5s increase the usable light output greatly. From what I have studied about reef lighting, more attention should be focused on reflectors (pun intended) than on the type of light, whether metal halide, power compact or HO fluorescent.
In my old 55g tank (18 inches deep) I kept an anemone for years under power compacts & regular fluorescent light (190 w total). It split lots of times, which I take as an indication of its health being good. And I never deliberately fed it, so I assume it got most of it's nutrition from the light. Palileo, where did you get the info you need MH for anemone? Specific type of anemone? There are other reasons to think long & hard before putting an anemone in a tank, (I would say "don't") but that is another topic entirely!
For your cube, if you can get a good deal on a used fixture with good reflector(s), go for it. If I was setting up another tank, I would use T5s with individual reflectors. They are available in 24 inch length.

I believe you could fit 6 across the top of an 18 inch wide tank. I do not think you would be disappointed!