Some couplers are adjustable, some aren't. Look for nut at the bottom of the spring below.
I agree that it isn't a good plan to use the wrong ball size. You might get away with it for a few moments, but you're playing with fire.
Having a little extra tongue weight will help keep it all together.
The couplers may wear some (the reason I like to grease mine).
I would like to see some oversized balls in case a coupler gets a little sloppy Say 2 3/8" or 2 7/16" to replace 2 5/16", yet that doesn't seem to be available.