What is the big difference between housing some corns together, and housing some ball pythons or boa constrictors together? The concerns/risks are the same, and the benefits are the same (unless you believe that garters are social creatures, which I don't).
I have two adult boa constrictors in the same enclosure, and three adult ball pythons in another. While I am not especially interested in breeding either species, both have produced babies for us (balls- clutches of 10 and 14 eggs, 100% fertility). I'm not saying that this is the ultimate way to keep them. But we have reasons for doing so that extend beyond not being able to afford additional cages.
Personally, I hate to see words like absolutely, never and always, especially with respect to animal husbandry.
I'm just saying, give people a little bit of credit, and space to do things a bit differently, without jumping on them for having bad husbandry.
Interesting note about the calci-sand- I've always just taken their word for it. I do have some hydrochloric acid here. I will check it and report back!