I have a Golden 500 with the Hydro-magic feature. I use the new twin blades in it because I tried it before I read not to. And guess what, they work great in it. I get some of my greatest shaves with it. Later someone told me it is ok to use them as long as you don't try to use the Hydro-Magic feature with them in it. Whatever.
The problem with the twin blades is that they don't have the two slots in them that allow the two pins of the Hydro-Magic system to pass through. Those pins are to retain the blade when the razor is opened. As long as you don't use the Hydro-Magic feature, you can use twin in the 500.