Dawlish and Sirius might have a lovely time not liking each other very much. Sirius is kind of critical of the Ministry and has a penchant for finding himself a bit of trouble (and a last name that has a habit of preceding him). They both seem to have a penchant for adhering to a first impression and such diametrically opposing views on life and authority that may set them up well for sustained (though perhaps almost cordial) confrontation?
As for Mary.... well. I might have an idea. But it might be my post-final scotch talking. So I'll just kind of ruminate on it for a while.