Well I installed all my pistons and my #5 piston is slightly cocked in the bore. I can turn it by hand, but it doesn’t change (it’s not any harder to turn than it should be). Does this mean I am going to have to pull it out? Could a ring be out of place? I’ve just installed the heads and oil pan yesterday. Will it mess up the gasket if I uncompress it and reuse it? Should I get a new one (it’s the expensive one though)?

 

Yes somethings wrong, you cant take a chance, pull it out and look again.
Unless its a copper gasket, which you can heat and re-use, never re-use a head gasket, its not really worth it.
You have gone to a lot of trouble to be plagued by doubt everytime you turn the key.