the petco where i live must be different, because some of my most long lived and beautiful and health fish have come from petco. in fact, i accredit the infestation of my tank with marine velvet to a specialty marine store (i learned about the value of QT
fast on that one!). just make sure you know what your looking for in a fish, no matter where you buy it...
i agree that returning the fish would be the best idea for everybody. the petco probably wont be able to take them back, but maybe the other fish store you spoke of could.
also definitly go with the liverock, if you get liverock from most places you have to let it cycle but that doesnt seem to be an issue with LR
i agree with lando, the fewer chemicals you add to your tank the better. natural is the best way.
good luck, keep us posted!