I just tried tonight. The new CXS 12 batteries will not fit an old C12, and the old C12 batteries (which are the same size as a C18 battery) will not fit the CXS 12.
Same for the CXS 12 battery in an 18V tool like the TID 18, whereas the old C12 LIon batteries will both fit in and run most 18V tools, albeit at a slower speed (the OSC18, on the other hand, will just beep at you for being so foolish as to try).
Ironically, the CXS12 battery DOES fit in the small SysROCK radio, and engages the terminals, and powers up the radio, but the tabs don't fully latch into place.
I use my old C12 LIon batteries for that radio more than I do for my C12, for better or worse.
For all of these, they slide/slot in just the same on all of the tools, it's just a matter of various moldings on the sliding profile to prevent or allow engagement of the battery on the tool (protecting 12V and 15V tools from using 18V batteries, etc).