On the rear derailleur, that J-shaped housing/cover.

Are those all about the same or is there any specific I need to watch for with those? Like length, insider diameter compared to my shifter cable diameter, how it screws into the rear derailleur, etc.?

I bought generic cable/housing sets for brakes and dérailleurs. I noticed there isn't a J-shaped piece for the rear shifter.

If I redo the housings, why not add a new J-shaped piece while I'm at it? Unless that's not done for some reason.

Between the J-shaped piece and the rear dérailleur there's a plastic adjuster screw piece. Is it worth swapping a new one in there too? I don't want to take it apart later and have to mess with adjusting brakes so it's easier to do it now. Assuming it's never been done before and the bike is now over seven years old, is it worth swapping in new parts there?