Each one has their own intricacies. It helps to know one, but it's not THAT much easier to learn another one. Everything from input rxs to locating the appropriate insurance to dealing with insurance rejects can be handled so differently that you can't really go from one to the other easily.
I've done RxConnect, RX30, PDX, and no IC+. Not sure which I like best.
Kroger has easyfill. I wouldn't worry about computer systems, just focus on getting a job. The computer systems will all become second nature once you work there a while.