Answer 1

No 2 dittos are compatable to breed but you can breed with any other pokemon that are not legendary. also manaphys. i hope this will helps you find out.

sources: read the breeding stuff on this site.

Answer 2

theepicME, you are wrong. Not all non-legendary pokemon can breed with ditto.

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‘Ditto’ egg group
Unsurprisingly, this egg group contains Ditto! Technically this is a real egg group but it's more of a meta-group. Ditto can breed with all pokemon besides itself and the Undiscovered egg group.

taken from the Ditto egg group page.