> is the mental model close enough to pick up without learning much?

I would say git core concepts are pretty similar to fossil concepts, but actual plumbing implementation details are pretty distinct.

The major difference that I remember from a day-to-day "git porcelain" perspective is that rebases and other types of history rewriting are very discouraged.

For a Rosetta stone of somewhat comparable commands: https://fossil-scm.org/home/doc/trunk/www/gitusers.md

> Can it sometimes use existing git repos

You can, but it's kinda a lot of really slow busywork. And you lose some of the not-file-dvcs features of Fossil, but it is possible.

This page explains how: https://www.fossil-scm.org/home/doc/trunk/www/mirrortogithub...