600 gallons per hour for a 450-500 gallon pond is fine. You should look into making a skippy or other similar biofilter rather than using a commercial one. They will get rid of your green water.

I always remove prefilters on pumps. They clog, stress the pump, I don't see their purpose beyond marketing.

There's nothing particularly good or bad about 600 GPH in a 500 gal pond. Depends more on filters and how you want to run your pond. For example, UV filters have flow rate specs. Bio filters can generally handle high flow rates, or I should say good bio filters.

In my experience I've never found one maker of pumps to be better than others. Seems to be more about luck. I did have one pump that ran 24/7 for at least 8 years, could still be running. It even got pulled out of a pond by a raccoon and ran for hours dry. But that's just a single experience, next pump from the same company could bust in a month. I've had that experience too.

The only sure way to eliminate green water is a UV filter. For a 500 gal pond they're cheap. And you normally only have to run them for a few weeks. That gives the bacteria that kills algae a chance to get established. Before that the algae produces a chemical that kills the bacteria. Getting that bacteria going in a Skippy is very hit and miss. Fine for nitrifying bacteria, not so much for algae killing bacteria.