The latest image is running buster so there is no reason it should not run on RPi, mine is running off a 3B using the GoogleAIY kit that came with the magpi. I’m working on making it a bit ‘smarter’. My daughter is keeping me on my toes with that!
OOTB, no (means with Picroft image). But you can copy the necessary files into /boot/.
8GB seems a bit over the top. Maybe if you want to go big on the gui side, but even a full fledged BigscreenOS (KDE Neon) is satisfied with 4. There would be some usecase in the Machine Learning departement… but mycroft itself? Not in the foreseeable future