Hello! I'm pretty new to TS3, thanks to this damn Steam sale I finally switched after playing over a dozen years TS1 and TS2. (I guess I will play TS4 when there will be obligatory VR TS5 or something).
I really want to have one particular Sim to be a mixologist, so I created him, cheated him max mixologist skill, and made him buy the bar. I thought the game will allow me to work there, just like business in TS2. Nah. He could only "moonlight" the bar. So, playing with a family I wanted to play, I selected an NPC bartender and used mastercontroler to edit them in CAS to look just like the guy I wanted. Always something. He would always stay the same and always work 11am-3pm, unless I rebooted the game. Then, someone else would appear.
Now, I think I have 3 copies of one family I put multiple times by mistake, and I thought I deleted all the unneeded copies. But then, after one restart, a lady from this family... showed up as a mixologist. My sim flirted with her a little, but I found out they weren't friends- she was not from "mine" copy of family. I searched all my neighborhood for the duplicate family I could delete- nah.
So if a Sim I created was able to become an NPC "naturally", does it mean I could make a Sim who owns a house in the neighborhood (but isn't in a selected family) etc. work as a mixologist in a particular bar? Or was she able to do that because she magically went to some secret magical NPC land when I deleted her?

Do you happen to have a copy (s?) of your sim in the bin (in the hood view, where all your bin families are shown)? Because the game LOVES to clone them when it generates NPCs! They're in no way related to that sim, they're only the physical copy of it.
IIRC If you use the Register mod from NRASS there's special settings that should let you define this playable sim as the barman of that specific bar object. Owning the lot do nothing except you collect money from it.

NRaas Register will allow you to specify any sim in your world as the assigned Mixologist Role Sim regardless of whether they are a resident sim or a homeless NPC. If you want an active household sim to work the bar without them becoming a formal Role Sim, use Register to undo and disallow assignments on it and constantly Moonlight -- that is the only other way an active can be a Mixologist (not counting a custom Rabbit Hole career where they don't really work a bar on-screen).

NRaas StoryProgression can be used to disallow the genetics and copies of specific households stored in the bin from being used for incoming immigrant residents, the SP Population add-on module can extend that to incoming NPC Service Sims and Roommates. The entire scenario of using the bin at all for such a source can also be shut down. We can help you with those settings at NRaas if you like.

http://nraas.wikispaces.com/page/messages/Chatterbox
(free registration and wiki membership required to post)

Thank you, hah, I feel like a total noob not noticing that :D

I tried story progression mod, but I felt overwhelmed with constant notifications about some random Sims i do not care about. Plus, I just got the game (though my Steam counter has already 200 hours on it ((but probably because the game loads so long I just leave it when I go to work so I can play when I get home)) and I don't know all "regular" stuff yet - a big mod like this really makes me overwhelmed at this point :D
Probably notifications are possible to disable, but then I feel like setting every family I don't want affected to not be affected would take ages. I use most other nraas mods, though, they're really wonderful. Thank you!

It's easy to dial back the Story reporting in one step rather than by individual household, although there is a way to do that too. The inactive sims no longer being reported on will still get the effects of those progression events but you just won't get the notifications.
NRaas > SP > General Options > All Stories > (select which sims by relationship to yours you actually want reports on, such as Friends and Blood Relations, or choose "None")

But I understand that SP can feel overwhelming to newer players and even to seasoned ones new to the mods. It's not really where one typically starts out, but it does happen to be the mod that handles immigration and hence the usage of bin households and their genetics to create new sims. The other way to manage that, of course, is to just not keep any copies of your households in the bin or at least none that you would want used for this purpose. That's less convenient but still workable for many.

Me too, no SP for me! To avoid the clones thing just never save your sims in the bin, save them in CAS instead! They won't be cloned there. (IIRC to save a sim in CAS: on the lower left click on the folder icon) The bin can then be used in the opposite way: create a household or 2 of very pretty sims, or very ugly ones.. Sims for your liking! Or even celebs... You'll then be happy to see those clones around, cool genes to mix with ^^