Brian why do you think it is that you have higher losses with more colonies in the yard?
I am glad to hear that diverse genetics is a good thing because I have NWC, Russians, Purvis Goldline and some unnamed mutts in my bee yard.
I think everyone has higher losses with more colonies in their long term yards. I thin 5-6 is optimum because beyond that number the following factors become increasingly hazardous: 1. Forage competition, 2. Robbing, 3. Drifting of drones and workers that spread mites/disease between hives, 4. Genetics, and 5. Hive Synergy.
With 5-6 hives you have or can have enough genetic diversity to have good matings but if all of the hives are of the same stock then the genetics can be a failing within the beeyard. Foraging is always an issue as the more hives in a location the less to go around, the more bees from other keepers within short distances compounds this problem. Hives within 1/2 miles radius might as well be considered part of your bee yard when assessing forage for your bees, in which case I actually have 15 hives in my bee yard. See the problem?
The more colonies verses foragable plants the more likely to invoke robbing--see the problem?
The downside of drifting bees where disease transmission can take place should be obious. Overloading a yard with hives will cause the loss of hives that might have otherwise survived because of reasons 1, 2,3, 4, & 5.
Hive Synergy is a hives own ability to use it resources for survival, the more crowded a beeyard the less this factor counts due to reasons 1 & 2.
I currently have Russian, OWC, NWC, and Texas hybrids in my bee yard, mostly from different sources in different years to preserve genetic diversity. Out of the 5 hives I will do a community split in June that takes 1 frame of nurse bees and brood from each hive plus 2 frames of honey/pollen. That gives the hive the remainder of the summer to build up for overwintering. I will be taking that split over to my brother's place in eastern Washington as he's my out yard. He has 5 acres in a land of acres of alfalfa and clover and I have 1.25 acres in the city. I visit him once a year and we swap plants, livestock, and commodities for sustainable living so we each have a more rounded operation.