I have always been under the impression (17 years I have had discus) that it was best not to mix too many different sources ( if at all) of discus in one tank. That a group of discus from source "A" will have a different composition of bacteria from a group of discus from source "B".
Makes sense to me, as our bodies are also comprised of bacteria/defenses or immunity different to other geographies depending on exposure.
I also understood that it wouldn't matter how long you quarantined a new group of discus, this bacterial difference would still be present, why would it change?

I was told today that this thinking is wrong: This is a ploy by discus distributors to pressure the consumer to buy from one source. That he, this person who I was having this discussion with, has been mixing discus for forty years and never has an issue because he "properly quarantines prior to adding the two sources together"
*I ask myself: What difference does that make in terms of the bacteria that will be on group A from group B?
This person says he has 7 fish in a tank from seven different vendors and the risk of cross-contamination is no more in this group than a group from same source.
Now, my gut says-- that is malarcky... but, I am not a scientist-- therefore, I could be wrong.
Can you give me a somewhat scientific reason why I am/or am not correct? or direct me to a thread where this question has been discussed before?