Well, my sponge filter's sponges fit nicely over the slitted filter intake on one of my Penguins so I simply use it as a prefilter. That keeps a good flow through it. You could also keep it in the filter box itself to keep it seeded. Most folks use as small a QT tank as they can get away with. I use a ten gallon because I know I can keep about any fish I buy in it with enough room for it to feel safe and secure without being too small.
If you remove the ammonia source from a tank, it will eventually lose its cycle. Most folks keep extra media around for Q-tanks, like my sponges or Kasakato taking stuff out of his canister. This way the filter media stay cycled and when I set up my q-tank I take some water from my main tank, fill up the ten gallon, slap the sponge filter in and I have a cycled tank. Most folks do it this way rather than leaving a fish in their q-tank constantly. Canister filters are great for being able to load different media in them and my next tank will be a large tank with 2 canster filters on it and I certainly plan to have extra seeded media for my q-tank, settnig up enw tanks, etc.