No matter the lot type when I place a pro bar and hire someone as a mixologist, someone else shows up in their place so I can't make them a custom uniform as I'm making it for the person I hired (who never even comes) as opposed to the person who showed up in their place. I even tried hiting idfferent people and the same thing happens. What do I do?

With Nraas Register, you can click on the bar and manually select someone else i.e. your sim. You may also need to use Master Controller + the Cheats module to add a Mixology skill of 10 to that sim, as the game sometimes doesn't assign the skill to mixologists, which leads to them not tending the bar.

The options to hire someone, set the shifts and edit the uniform were added to the game by the Island Paradise EP/patch. I don't think that they were intended for bars outside of resorts and they seem to conflict with the system that is in place for Late Night bars, Showtime venues and other community lots that allow mixology bars. It was a nice idea to add this to all bar objects outside of resorts but I don't think that it really works.
I, too, always use Register now and the Sims I select with Register come reliably and they keep their role. But this also means that I'm stuck with the uniforms the game assigns.

I see. Thanks for the info. Do you guys kno wif my theory that you don't have to pay them is correct as well? I'd like to choose their uniforms still though. When I tried to set the other guy's custom uniform it had me edit carer. If I edit the NPC given to me's career outfit with Master Controller will that work?