You get full res on any device. You just have to pay if you go over 15 GB.

No, free, unlimited full-res is exclusive to Pixel, otherwise full-res photos count against the online storage quota (I think there's 15GB free, and additional storage available for a monthly fee.)

Google's original 'unlimited' was 16MP, and the pixel makes 12MP images, so wouldn't it be within the original limit by default.

If the pixel makes 4K videos, then that would be over the original unlimited limit.