anticapitalista
You have to choose one or the other since slim cannot do one of the things you want it to do and lightdm cannot do the other.
If I were you, I would stick with slim and remove the user login name in /etc/slim.conf/default_user xxxx. Then all users can login (after applying their username and password)
Philosophers have interpreted the world in many ways; the point is to change it.
antiX with runit - leaner and meaner.