It doesn't have to be the exact length of the tank. It can be shorter, in which case you put your higher light corals in the middle and lower light mushrooms and stuff on the sides. I prefer Current USA brand. I have a 4ft and 3ft compact florescent strip which I've had for years without issues, and the 3ft even got dropped in my sw tank at one point. I notice they don't have the t5 under 18", but they do have the CFs
http://www.current-usa.com/lighting/sunpaq-lamps
And don't be scared away from CFs. T5 are the new trend, and some people try to say that t5 is better in a significant way, even trying to say CF don't work adequately (I believe these people haven't used them) but my high power 4ft CF light will actually provide TOO MUCH light for a large variety of corals, including many LPS unless you put them on the bottom in a 90g, so I don't believe that. The reflector is a major factor in the efficiency, and current usa has great reflectors. CF bulbs have also come way down in price.
I do suggest staying away from Coralife fixtures. I've had all 5 of my Coralife fixtures fail, and they weren't all the same model or length, either ballast or switches, and none of them lasted over 2 years.
Or you can do something like an LED par spotlight, one of which would be fine for a 10g. Something like this
http://www.rapidled.com/12-led-par38-bulb-mixed-color-with-uv/. I had another site bookmarked that had good reviews, but it's on my other computer and I can't think of the name now.
