Can I safely use a 7.5V 600mA adapter on a device that requires 7V 500mA?

From an engineering point I can't say. From a real world point (experience) I would say you are fine. Depends on the device.

Adapters vary a bit but I have used 10VDC on 9 without any problem.

Thanks guys - will try and if it explodes will get back to you !