500 I would say, yes it can be sailed. But 500 lbs is a lot of weight. That would be equivalent to three medium frame persons. The 4.9 is only 16 feet long. You will be pushing you limits on space.

600lbs is really pushing it now. Why so much weight?

Regarding to pitchpoling, that has more to do with the angle of sail. Of course if you put all 500 lbs on the sterns I dont think the boat will pitchpole.