You can still seed the tank even though there are fish in. If you have another tank, use stuff out of there. If you don't, find a friend or even a LFS that will loan or give you some gravel or filter material. Will definitely speed along the cycle and make your fish happier.
I use this all the time for my quarantine tank. It is usually stored dry in a closet somewhere until I need it. When I set it up, I take a sponge from a filter on one of the currently running tanks and put it in the filter for the QT. On day two, I get a bacterial bloom and the water goes a little cloudy. On day three, the water is clear, and the tank is cycled. No muss, no fuss.