If you are going to neck-size only, generally recommended for .303, keep your brass segregated by rifle.
I have never cleaned my .303 cases, bit I will sometimes clean the primer pockets if they are especially dirty.
You can full-length resize the orphan cases or just neck-size and be prepared for a hard feed or two. They will then form to that rifle.
The .303 round is pretty easy to reload, and you'll like the results.