The legs on all of the fixtures are adjustable. Also, yes a 24" fixture is what you would want although I would like to steer you away from PC lighting and try to convince you to pick up a T5 or Mh fixture. You can get a 24" T5 fixture for approx the same cost as the PC fixture. Take it from me first hand, I was running a 2x65w PC fixture on my 20gal (6700k/10000k and 420nm/460nm actinic). I made the switch to a single MH HQI fixture (150w) and was BLOWN away!
I am not trying to preach to you but reconsidering for T5s or MH will please you much more in the short and long run.