The smallest T5s fit perfectly over a 20H. Personally, I would suggest you save yourself some time and just go right for the MH. Personally, I think a single 150W 14k bulb would be all that you'd need. You could either get a retro kit and make your own hood with just the MH, or get one of the smaller MH/PC fixtures on the market. If you get a fixture with flourescent lights, then you could do a 10K MH bulb since you'd have the actinic supplements.
As far as the clowns... good luck. Over time, my ocellaris have hosted with a powerhead, a rock, a piece of eggcrate, and an oyster. This is the same pair of fish BTW. They have ignored the BTA and other LPS like torch.