In my view, it's not the distance between hives which is important, but rather the distance between entrances. So - nuc boxes with a single, central hole can be placed physically closer, than (say) brood boxes with wide slots. The sub-species of bee is also a factor, with Italians being more prone to drifting than Carnies.
With full-width slots, I'd go for 18", with single, central holes 12" - both within your spec.
If you can point the entrances in different directions, then place the boxes as close as you like ...
My telecovers often touch each other, so that's about as close as you can get. But for easy of working I prefer more butt-room b between the hives. I doubt the bees care much. In Europe, it's not uncommon for some hive setups to share common walls on a specially-built trailer.
All my entrances face the same way, BTW, whether centered or reduced with the reducer in place.
Sometimes I speculate that my unusually calm yard benefits, collectively, from appearing to strangers outside the yard as being a single really, really, big colony not worth messin' with just to steal a bit of honey. I have mixed-sizes of hives within the yard (from a nuc, to three-deep wintered-over Big Girls) though. I also protect most entrances nearly all the time with robbing screens, which are identical in design. A quiet. unfussed, "happy" yard is one of my main goals.
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