Flushing radiator, must i take off thermostat?

Removing the thermostat is not necessary. Just make sure you use a good quality radiator flush and bring the car to operating temperature so that the flush goes through the heater core. Prestone used to sell an adapter that you splice inbetween the heater intake hose and you attach a garden hose to the fitting and power flush the stystem. I have one on my car, and never had a problem.

In most cars, water is going through the heater core anytime the water valve (if equipped) is open.

As soon as the cool flush liquid hits the thermostat, a properly working one won't remain open.

A better idea is to pick up a new gasket and thermostat and just change the cheap little thing if your radiator has been used long enough to actually need a flush.