AFAIK, everything about TW in 15 is similar to 14. Just as 14's BIM Server is a modular server with an optional 13 and 14 modules, in 15, the modular server for AC 15 can have any combination of the 15, 14 and 13 modules, installed, supporting projects in any of those versions of AC.
Just as now, everyone on a TW project must be using the same version AND build. So, you can continue to use 14 with the 14 module on the 15 BIM Server to finish those projects. Or, you can migrate projects to 15. But you cannot mix versions.
There has never been a requirement that everyone be on the same platform (Windows or Mac), only the same version and build.
You will not find anything about any quirks concerning the interaction of 15 and earlier versions other than if Graphisoft encounters some that will be put into the release notes. Any further quirks will no doubt be posted here rapidly by anyone who finds them after release.
If you study/experiment with the 13 and 14 modules within the 14 BS, you will learn all you need to know about dealing with the 15 BS.
Cheers,
Karl
AC 28 USA and earlier • macOS Sequoia 15.4, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB