There used to be a concern that the gassing action of mixing the akaline developer and the acidic stop bath could cause pinholes in the emulsion. For this reason, I learned to just use a water rinse between developer and fixer. Been doing it for 60 years, and my father 30 years before that, and it works fine.
For prints, especially fiber base paper, stop bath should be used, as the paper has soaked up a lot of developer. While this won't hurt the process, but it significantly shortens the life of the fixer.
So far as how much developer to use - yes, enough to cover the reels is necessary. You many want to consult the developer instructions, as some list a minimum quanity per roll. To little and full development won't take place as the developer becomes exhausted.
As the advice above, powder mixes should use the entire container. With something like Rodinal, just mix what you need. I usually use the one shot method rather than replenshing. I think it gives more consistancy, and mixed developer doesn't sit around oxidizing for weeks.
Just IMHO 😎