I can't speak for the 5 I do know my mini 3 standard battery stopped working in my 4 pro after a FW update. The plus and the 4 standard battery worked in both so how the 5 battery will play along with a 3 or 4 remains to be discovered.
I just read about over 250g weight issue as well. OOPS!
My mini 4 pro is registered (as was my mini 3 when I had it) so I could fly legally with the plus battery but at least they were legal with the standard battery. This is not so with the 5...