Hey there, sounds like your tank has a pretty good sized community already. Removing the dottyback will definitely be less stressful for everyone (or alternatively you could remove the cardinals).
There is lots of activity in the buy and sell forums so you should find some good deals on what you need and yes people do buy and sell fish from home aquariums. Sometimes I find that is better as the fish are in good condition and are already eating prepared foods.
Keep an eye on your fluval filter, lots of detritus can build up there and cause problems with nitrate levels. Its fairly easy to clean and if there is a sponge in it, give it a rinse with some old tank water during a water change to get the buildup out of it while maintaining the beneficial bacteria
