I just put a regular Pidgeot and a Shadow Pidgeot in the Daycare,
right now it says "They seem to match perfectly", so breeding
should be possible in theory. I will update you if I get any eggs
:)
(Though, you can not get shadows from eggs, as stated in the wiki)
Sooo at least regarding Pidgeot, crossbreeding Shadow and
non-shadow works. I successfully got Pidgey eggs from my mixed
pair. This would mean that Shadow Ditto could replace a regular
Ditto. But I can't test for sure as I do not have Shadow Ditto :)