I doubt mixing metal impregnated anti seize greases on threads has much, if any negative effect. Because if it did you would be hearing more about it. With the millions of vehicles out there, most of which are serviced by different garages / dealers / mechanics, all of whom use whatever anti seize they have on hand. Be it Zinc, Aluminum, Copper, or Nickel. If there was some type or form of negative electrolysis / erosion / conductivity taking place, it would be a bigger issue. It's not an issue at all. But you read about seized / stripped / frozen / broken nuts, bolts, studs, and spark plugs everywhere.
Personally, I believe having different types of anti seize applied, is better than having none at all. No, I don't have any, "scientific data" to support this. But again, if it caused negative issues in any great abundance, you would be seeing and hearing the negative results, posted in automotive forums like this one. You don't. And having a bunch of people saying don't do it, isn't saying much if they don't have the negative results to support why. Bottom line. If you've used copper in the past, and you want to use it again, go ahead. Nothing bad is going to happen.