kelvin rating is the color temperature of the lamps, it has a light effect on the brightness, but we don't worry about that. as for the wattage the longer the T5 the higher the wattage. all T5s will give you the sam light output in a messured area, IE. in a 6" length of a 24" and a 48" lamp they will both put out roughtly the same light output.
as for putting a 3ft ligth on a 4ft tank , you will be loosing out on a foot of aquarium for higher light corals, it will still get lite up but not well