So two T5 30 inch fixture with a 24 inch bulb? How high should it be?
If the fixture you bought come with a mount then that's the recommended height. If not then about 6 inches off the surface of water on a 20L.
As for T5 bulb get 6700 kelvin and 10,000 kelvin. Shrimp whiteness (CRS) show off their color better in 10,000 kelvin color. But be prepare to scrub off algae if you don't like it. I like aesthetic looking tank for all my tanks, algae is a like a disease in my eyes.
If you could get a timer for your electrical outlet for the Light fixture. Mine set up as follows:
9 AM - 2 PM ON
2 PM - 5 PM OFF (hottest time of the day during summer)
5 PM - 8 PM ON
8 PM - 9 AM OFF (fishes and shrimp need their sleeps)