But it may also be that the tank is not as oxygen rich as we would like. And the big guys might be going through a little shock.

Long established community tanks often have accumulated their own unique chemistry that will really jar new residents. I couldn't complete the enumeration all of the factors, which must be considered in the old water or the old tank. Again, as gloriously empty-headed as it sounds, please continue a regime of increased partial water changes. That will tend to dilute what-ever characterized the community tank and if the new water is appropriately treated, seasoned and warmed to tank temperature (as I know you would do) that should ease the transition for the newer fish. I still may encounter a loss of an old-timer which making such migrations. Those things still just happen regardless of what we do. :(

For instance, I re-did a planted 40-gallon aquarium, which had been set up for over a decade, ok, really two decades. I guessed that the total TDS, the nature of whatever was in the gravel and the combination of ions in the water were all too high and not likely to be what the newer residents had "experienced." Over a week, several good sized, partial water changes and a lot of gravel vacuuming took place before new residents were added. And an 8-year old yellow rainbowfish (a magnificent "herbie") still leaped out (a sign that they are not happy or are fighting with somebody, I have a candidate) when we weren't looking! :(

It might have been better to remove all of the rainbowfish residents and rearrange that decor. Territories would have been obliterated. Of course, the nitrogen cycle might have been at risk without active, metabolizing fish, after a while.

Your sinking of the food which would reach the bottom dwelling males is clever. I'm sure with your energy and resourcefulness, that the vast majority of those guppies will make it.