Is there a way to pick coworkers?

I don't think you can. When you select employees for restaurants and retail stores, you can only pick the random sims that show up.

For normal careers, you can cheat them into the job but you have to go into their household to do it. I do this around my entire worlds so that everyone is working in the jobs I want them to be in. I have always found them to keep their careers and job levels as I rotate around different households.

Eg. say you want Bob Pancakes to be a doctor (lol), you go into the Pancake household and click on Bob and get him to quit any job you don't want him to have, then join the one you do want him to have via the phone. Then open the cheat box with Ctrl+Alt+C then type in: (I think you need testing cheats on first, actually can't remember) -

testingcheats on
careers.promote
repeat that last line until he gets to the level in his career you want him to be

so, for the doctor career you would type in:
careers.promote doctor

If you want to increase the skills to match up to the career level you can do that with:
stats.set_skill_level

so, if you want to give Bob level 9 in logic you would type:
stats.set_skill_level major_logic 9

(adult skills have the word major and an underscore before them)