I've got a 50 size helicopter I bought. It requires 600 mm blades (not included). I have a pair of CF 550 mm's. Will these work okay? Or is it a problem?

It will work but you risk over speeding them. You also wont be able to put as much torque to the blades as with 600mm but if you keep the throttle curves under control they will get you by till you can get some 600s

Maybe set 80% throttle at each end of the curve in idle up and watch head speed with an optical tach