Tom, I was forced to combine my 3 domestics (Kenny) and my Wilds due to lack of tanks. They were separate for about 6 months with no intention of combining them but a pair changed all that. I was really apprehensive about combining them. They were fed the same, WC the same, same water, etc. All things were well and no illness. When I moved the fry to the 40g, I combined them into the 75g.
The end result was no issues. They are generally the same size and I guess approximately the same age. The 3 domestics (which were a little “friendlier”) gave the Wilds a little more security. Now, if a pair begins to start from the Domesitics to the Wilds, I would discourage that. But, no issues so far.
What sources are you looking to combine?