I did a quick google about marrying NPCs and found a list of which ones were and were not safe to play, and I believe the cashiers are. There was this pretty dang cute cashier named Phil McArthur that my game generated and I'd like to keep him in mind for my toddler Stefania because he would fit in with the overall family aesthetic really well.
But I know that normally you can't call/hang out with cashiers, bartenders, etc and since he is a teen I'd like to send him to University with Stefania when she's older, but I'm not sure if he'll be able to or not because he's an NPC. Would I need to cheat him into the family first and then send him? Or will he be in the thing to send to University anyways?
Also, slightly unrelated, but I have notownieregen and anti-redundancy in. If I do move him into the family will the game generate someone to replace him? Many of my community lots have two registers so there's two cashiers in the town at present. I've never had any interesting in moving in an NPC before to know how that'd even work.
This is not a problem. If you go to "Send Teens to College" in the University neighborhood screen, you can call up a list of eligible teens and can select anyone on the list for a college household. Once you indicate that your household is complete, the game sends them to the student bin, and you can move them into a dorm (or residence, if they can afford it; default cash for a teen with no scholarships is $500), where they will arrive in the taxi and grow up just like playables and non-NPC townies.
All the teens accessed from the neighborhood screen are safe. There are no known problems with any teen type.
You can normally interact with cashiers if you see them passing by on community lots where they don't work. Or you can talk with them in shop and invite them later over and they are totally normal teens when they arrive. My sims are often marrying ex-cashiers when I send them to college together.