The fact that they are different species isn't an issue. I'm able to maintain proper husbandry for all four, even when they are grouped together. A bookshelf would be much more expensive than the plastic shelving, and since it's only temporary, cheaper is better.

I'm not planning on "getting new babies in", so there's no sense in building a rack with the intention of using it for babies when the adults outgrow it.

As for moving shelves in a rack, the plastic shelving unit I'm talking about would accommodate many different sized tubs, no building or moving involved. My main concern is its stability, though. I don't know how sturdy they are.