Recommended PSI for 16 Inch Wheels?

You still use the car makers recommended pressures.

The sidewall pressure is the safety limit and not a working pressure (eg. it's guaranteed not to burst below that point).

Note that

a: it's illegal to drive with under- or over-inflated tyres,

and b: your insurance is void if have not told the insurer about the change to the vehicle, as it's no longer a 'standard' production car.

If they're non-standard wheels & tyres there isn't a recommended pressure.

Use about 30psi as a guide, experiment with 2-3 psi under/over and see if you can feel a difference.

There should be a sticker on the edge of the drivers door that shows "recommended" pressure.

You can air them up to whatever pressure works best for you, just don't go past the max rating on the side of the tire, which you said is 51 psi.

35 sounds good to start with, you can always adjust as needed.

The wheels don't have anything to do with tire pressure. The factory recommended tire pressure in inside the drivers door. Read the maximum tire pressure in the tires. Add it to the drivers door pressure and divide by two.