The 8050 Di 2 RD mounts exactly the same way as the 6870 RD does, just get your 5M allen wrench and screw it on. What the 8050 does is separate the MOUNT from
the PIVOT allowing larger cassettes. This separation of the mounting bolt from the pivot point has been standard in ATB RD for several
years, necessitated in part to accommodate up to 42t in the rear for 1x11 setups. Shimano calls this Shadow RD. 8050 users elsewhere
have used 11-36 cassettes without difficulty with the GS version. Road RD prior to the 8000 series had the pivot points and mounting
point at the same place.

As to mixing 6870 and 8000 series Di2, best look at compatibility tables Shimano provides. With the 6700 -> 6800 change the only
6700 component usable with the 6800 series were the battery and the brifters (well cables and dumb terminals also)