That would work, just make sure you get the freshwater one. It has the bulb for plants in it already (6700K).
The bulb is the same length in the 24 and 30" fixtures, so its not like it would cover any more length over the tank. I'm not sure if the legs would stretch that far, if you got the 24" fixture. My suggestion would be to get the 30" fixture. Doesn't look like the Drs carry it, but Hellolights.com does and its cheaper than the 24" at Drs F&S.
link:
http://www.hellolights.com/index.asp...ROD&ProdID=213
I've had excellent results buying from them, just FYI.
The legs will fit with the 30" fixture on the 30" tank....I have a 30" fixture on my 29g. I wouldn't lay it on a glass top....the heat produced by the fixture and not being vented (lying flat on the glass) could crack it. I've heard of that happening too much to tell you to try it.