Over my 20+ years of keeping fish I am now seeing much more disease coming in with new fish than previously. And ich seems to be getting more virulent, even causing fish deaths which at one time was highly unlikely if caught early. So quarantining all new fish for 3 weeks minimum, preferable 4-5 weeks, is a very sensible precaution.
The safest method to deal with ich is simply raising the temperature to 90F for a week or slightly longer. But not all "tropical" fish can handle this. But there are other safe methods I won't get into now.
To your question on fish for your 55g tank. What are the water parameters, meaning GH (hardness) and pH of your tap water? Some fish have specific preferences, others are adaptable to some extent. But knowing your source water and selecting fish suitable is always less fuss and more likely to succeed. The GH (and pH) you can get from the water supply people. A good pH test kit, like API's liquid, is a worthwhile investment. The GH will not change unless targeted, so once you know the tap water GH that is where it will stay.
I will end by mentioning our profiles, second tab from the left in the blue bar across the top of the page. Each has info on water parameter ranges, minimum tank sizes, numbers of fish in the group where this is essential, compatibility issues, etc.