The sad thing is that not many folks, even at Mossberg CS, know enough about what the Flex system is or does.
I'll back up what's already been said, the receivers, barrels, and mag tubes are the exact same on both standard and Flex models. All that's added is a forend and stock adapter, both of which can be removed to convert the shotgun into a standard non-Flex model.
What's likely adding to the confusion is that Mossberg doesn't sell an 8-shot version of the model 500 with the Flex system. All Flex 500's are set up in the "field" or "multi-purpose" configuratons with the 5 round mag tube and the Flex 590 models all come configured in the 9-shot variety. Therefore some voodoo must exist which prevents one from configuring an 8-shot Flex 500.
Bottom line, you should have no issues reconfiguring your 500 with the "hi-cap" kit or using your Flex forend set-up on the new mag tube. Keep us posted if you make the swap and we'd love to see some photos of your custom hi-cap Flex 500...