I would hook it up to your 55 gallon, you could never have too much filtration. The only thing is make sure is doesn't create too much turbulance in your tank where your fish will have a hard time swimming.
Water changes, water changes and more water changes. With the overfeeding and the addition of more fish than should be in there water changes of 50% every other day should start lowering that very high Nitrate reading.
Did you quarantine the new fish that you got before adding them into your existing tank. Eventhough your tank levels are normal nothing says that the fish that you added didn't have any diseases.