Bleach. Fill the tank with new water, add 5 cups of bleach and anything you are going to be reusing in the tank (filters, heater, etc.), plug in the filter and let run over night. Ditch any filter media and start with new, also ditch anything porous (driftwood, sponges, etc.). Drain/refill a couple times with fresh water and let it dry. Your stuff is ready to use.
Wouldn't recommend using the test kit unless problems arise, but do what makes you feel better about the tank.
Two Aquaclear 50's is a bit on the weak side. It's fine, but 1 AC 110 or 2 AC 70's would serve you better.
If you only plan on changing water 2-3 times a week, it's probably a good idea to cut feedings down to twice a day. Make sure you're changing a lot of water when you do.
You mentioned selling them when they get bigger, but that sounds like a bigger pain than tearing down the wall and plopping a bigger tank in there.