This is what OFL told me to do - put the sponge filter running in an established tank for a couple weeks, which is the time it'll take you to get your pair and condition them, and then just move that sponge filter to your fry tank. Boom, instant cycle. I didn't turn my sponge filter on until the fry were like 5 days old, and just removed water every day, just a little, sucking up waste on the bottom. Then I started adding.
I know people use 5 gals but it will be too small for growout, so if you use a bigger tank, the less moving fry around you'll have to do, which lots of people prefer as fry can be fragile. At the end of the day it's up to you and what you feel like is going to work.