It's "Those who work at university are smart." or "Those who work at universities smart"?
Completely different. The first is a statement about people working in a university job being smart. The second is a statement about workers being in pain.
Presumably that's not what you meant, so let's change the latter to "are smart" rather than just "smart".
There still is a difference: a garbage collector (recycling assistant? Don't know the current PC term) or construction worker may very well work at universities but not work at university: "working at university" implies being employed at some university, "working at universities" only means working there (possibly at several different universities), but the employer might be somebody else.