sds +/max interoperability

No, they are completely different. I wouldn't be surprised if an adaptor is available, but I don't think you'd really want to use an sds max for a 6mm hole - way too big and bulky.

I've used the opposite adaptor, i.e. to use an SDS max bit in an SDS+ drill.

Do you remember where you got it from, Andrew? My dewalt sds plus drill came with a sds-plus-to-conventional-chuck converter so i'm hoping the sds max drill will have a similar thing in the box.

Hire shop lent it to me (free) when I hired a 40mm SDS max bit. You also need a tool to separate the adaptor from the SDS max bit because they intentionally jam together. In my case, I simply took the two back and they separated them, but if you are hiring more than one SDS max bit, you'll need an adaptor for each bit unless you have a separating tool.

I've never inspected an SDS max chuck, but I presume it either works differently from this adaptor, or it includes a separate tool for taking the bit out after use. Certainly I doubt it works the same way as an SDS+ chuck, as the back end of SDS max bits seems to be slightly conical, so I doubt they are intended to slide in their chuck. The adaptor had a similarly conical hole to take the bit (with the SDS max 3-way keyway), and had no additional mechanism other than it jamming in the hole to hold the bit in place.