The dip is more than likely unnecessary since the quarantine is a preventative and not a treatment. The QT tank is meant to discover IF anything exists, not to stress the fish to the point of creating a problem. You might want to actually quarantine the fish for 4 weeks since two weeks isn't really adequate to discover diseases, let alone the latency period of ick is approximately 30 days. Also, the methalyne blue isn't always good on all fishes, and the same goes for the fresh water dips...

As for ammonia spikes, you shouldn't have any problems if you are using the cultured sponge from the main tank and you don't overfeed. Also, remember to keep the items in the tank to a minimum and with NO substrate. You should only have a few items for the critters to hide and sleep in.