In my opinion you go back and forth with the purchases. You start with some clean up crew, add a fish or two, add more clean crew that may now be needed and when they have things balanced again you get another fish and start again.
You have a 55g right? In mine, I have 2 scarlet skunks that stay practically glued to one another. I have 8 peppermints. I started with two but rarely saw them so added 6 more. 2 have taken a ride down the overflow and are living in the sump however. I think sexy shrimp are cute but they are so tiny I opted not to get them for fear I'd never see them.
Porcelain crab all the way, you can get more than one by the way. They are reef safe (filter feeders that do not hunt with their claws). Keep in mind that Anemone crabs are sometimes mistakingly labeled as porcelain crabs but they are also reef safe. On the emeralds, the troubles with them tend to occur when they get larger and I've seen video of one even attacking a yellow tank by dropping on it while sleeping much the same way a green brittlestar will.