Should PEX connections rotate?
Yes, this is normal, and true of both crimp and cinch style connectors. It's also a useful property of PEX during installation, as it makes it even easier to dry-fit and pre-assemble sections and then fit them into place.
Can the connection be used as a union?
A union is designed to come apart, and in copper or PVC plumbing are typically used in two situations:
- To make a part accessible for removal for servicing/replacement (such as an in-line filter)
- In a tricky installation where there's no other option for the final connection to be made
In the first case, a simple PEX connection obviously doesn't satisfy the need since it doesn't come apart, and so you'll need an actual union.
In the second case, the flexibility of PEX and fact that the connections can rotate does allow installations that aren't possible with copper or PVC. I've installed dozens of water treatment systems (typically involving complex turns and tight spaces) and cannot think of a situation where an actual union was needed because of the install. But if this is what you mean, then I'd say yes, you can avoid using a union and just rotate the PEX.
If you are trying to do something that involves regular rotation, where you'd normally use a rotary union, I'd say no -- you should just use a rotary union. The fittings aren't designed with rotation in mind, it's just a property of the style of connection. I'd expect if used this way, it's going to wear out the pipe and/or connector itself and the connection will eventually fail.