What is this mystery pathogen that will occur if you mix wilds and domestics?
OP, if you quarantine your fish properly and both groups are healthy, this should be possible. I mix wild cichlids with tank-bred cichlids all the time. Discus are no different. They do fall prey to pathogens more easily than some cichlids, but if you follow strict quarantine procedures then it shouldn't be that big of a deal.
Personally, I agree. And my past experience keeping discus also agrees with this. That said, and to be objective about the information that's out there, not mixing discus from different sources or not mixing wild and domestic discus is common advice, including some respected sources like Heiko Bleher (
example) or Stendker's German web site (
here-- or
here).
But, for example, in the Bleher article, he suggests a fairly wide variety of other fish (including various cichlids) to keep with discus and these include angelfish. And how many thousands of hobbyists are keeping their discus with all manner of other tankmates without a problem. Certainly, cross species infection is just as possible as same species/different source infection, it certainly is with nearly any other fish. Yet how many thousands of hobbyists keep discus with a variety of tankmates or how many are blissfully ignorant of the advice not to mix discus from different sources and never have issues?
So, imo there are two sides to this and you'd have to weigh it out for yourself. Really, there are divided schools of thought about a lot of things regarding discus, including water, tank setup, tankmates, feeding, filtration, etc... can make things confusing. But also imo (and experience) healthy discus is more about first getting healthy fish from good sources, then keeping them in healthy conditions (good water, good redox, good nutrition and sensible feeding practices, etc.) than some of the restrictive ideas out there about what you can keep them with.
...just my opinion