Should there be a washer on the copper nut thing that i tighten up to
the plastic pipe or not? - I didn't look to see if there was one before
so without taking it back off i don't know.

Yes, there should be a washer. It is probably a 15mm tap connector washer.
It is probably attached to the old valve. You should get a new one anyway,
as the old one will be manked.

Be very careful with metal tap connectors on plastic tails, it is
exceptionally easy to cross thread.

Christian.