Using a skimmer can help keep nitrates down since it physically removes some organics from the water column. Sounds like you have a classic case of new tank syndrome, and waiting and patience is the best cure.
Anytime you add fish you run the risk of introducing the ich pathogen. Normally happy well fed fish are able to seal with it on there own without obvious outbreak, but stress, and poor water quality can definatly contribute to its making an appearence

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