Your filtration sounds good, your just going to want to test the water around twice a week to make sure your levels stay in check, i.e. nitrate, nitrite, etc. If you start to see a spike you will probably want to check the filters and see if there is any significant buildup that is out of the ordinary. The damsel could probably be traded for a chromie. Chromies are just as hardy as a damsel and are non-aggressive. That is probably your best alternative right now. If they dont have a chromie, ask what other hardy fish they have and go with that. Anyway for now you might want to put him in the tank until you can take him to the store. IF he is aggressive immediately, I would put him in the bowl and let nature do its thing. If he lives till tomorrow, and your able to take him in so be it, but if not o well. I understand its cruel, but it is what it is and Im probably going to catch some flaming for that but o well. The mysis is most likely contributing to your ammonia. Basically all new tanks will have ammonia and the mysis is just aiding it. Flakes/pellets are much less messy as they cant disperse like mysis can. Dont get me wrong, mysis is a great food that should be used, but in this situation I would see if the clown will eat and if so feed him flakes/pellets. If he wont eat flakes/pellets I would feed him the mysis just so he will eat. How is the clown looking right now?