Generally it is not ok. I'm not electrician, but that rating is there for reason.
If you use lower rated cable it doesn't mean that it will supply less power, but that your device will try pull for example 10A through cable rated for 6A, which can cause overheating and possible melting or fire.
But to your specific case. If you use 500W PSU it should draw certain max power from outlet.
Equation is simple A x V = W that mean W/V = A
Or in your case 500W/250V = 2A.
This greatly depends on actual voltage and actual power drawn from outlet (as this may be higher or lower than rated power of PSU). But even if you would draw 700W / 220V = 3,18A
So that 6A cable should be able to handle it without problem.
But for better safety I would rather have cable with higher overhead for possible spikes or if your PSU decide to go crazy and draw way more power.