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IIRC the configurable diff tool is just to show diffs to the user, not for internal storage. So everyone on your project can use a different diff tool without problems.

An analogy would be like of the editor. Think of this tool as the editor that one uses. It does not matter to git what editor one uses; similarly, it does not matter what diff pager one uses :)

Yes, you can turn it on without having any side-effects for others.

You can use difftastic as your default git diff tool, but you can also use it as an opt-in diffing tool. I recommend using it as an opt-in, but defining a git alias so you can do 'git difft'.

https://difftastic.wilfred.me.uk/git.html has docs.

You can turn it on safely. Git does not store diffs (like a diff chain), but a snapshot of the files at each commit