I know that you can get lots of skills on NoSQL databases using a single Linux machine, but eventually you need to pick up skills on a horizontal cluster running the NoSQL database(s) of your choice. I am thinking of creating a personal lab. Would Raspberry Pi's be suitable. What are some other suitable mini-computers? 

I hate calling these little guys minicomputers. Minicomputers run things like AS400. They are the next step down from a mainframe. I think they should be called centimeter computers since they are on a centimeter scale. Millimeter computers would amount to a Raspberry Pi on a chip that was less than say, 90mm square.