Legs would still work, just place legs at the corners of the pico.however 1.5" of fixture is going to be hanging out.(in each side)...
No, the tank and fixture dont have to be the same lengh ..
fixture can be shorter or longer than the tank's lengh & vise versa..Lights should be Just long enough to cover more than half space of tank frame, but not so much that its way longer than the tank's frame, to avoid light being wasted...
You can also build a "V" shaped canopy (measuring 6" at the bottom and 9" at the top) to seat on the pico's frame and house the fixture..you dig?