Assigning NPC roles

Unfortunately, no. Your best option is to wait for the game to make those Sims then edit them in CAS to be what you want.

Yep, editing existing ones is the only way. If you save ones that you've already edited to your library, they should also work if you place them in other saves (as long as you don't put them in homes or mark them as played)

Sometimes my sims that have the active trait show up as gym trainers, maybe that can help you.

Really? That's kind of annoying, but I appreciate the responses.

And also for cookers and mixogolists, I can choose my mixology or cooking skilled sims by the interaction "hire specific caterer/mixogolist" for 100 simoleons.

You can add the traits/careers using cheats, but once it is done you can no longer play those sims.
Most of the cheats are listed here (It's a link to a mod, but the cheats do not require any mod) http://modthesims.info/download.php?t=595445

I don't think so without modding but what you can do is edit the townies that are already assign that role to how you'd like them, you may well already know that though. You'd probably need to use full edit mode cheat to change their personalities though, but at least it's mod/cc free for those that don't use them. Such as I.

I wish we could do this. I have a whole thread about it, been around for a while, but no change in the game yet to let us do this. :(

"Terra;c-16485853" wrote:
I wish we could do this. I have a whole thread about it, been around for a while, but no change in the game yet to let us do this. :(

It would be so much easier to set up a new save.

To be honest I'd like more functionality in manage world in general, it'd be nice if you could change a townie family without actually entering cas, like if you have your gallery filled with sims you are going to populate with.
More realistically I'd REALLY wish it didn't reset the filters every time you finish editing a family, or at least have a "show only" so you dont need to uncheck all the areas every time .-.