Could I put some swhwalbe city jet tyres on my bike for the road.Have Mavic D521 rims. I was looking at the 1.50 tyres. Too thin?

I think you will be fine.

I have a Bonty 1.5 slick on a similar width old-school rim and a Hutchinson 1.3 Crosslight on a wide rim too.
Both are fine and have done many road miles without incident, burping or punctures.
I run both at approx 60-70psi.

PaulD

A friend had a 1.25 Spesh Fatboy on a D521 as a turbo train wheel, was fine!