I have a magus with a cassisian familiar who took the Mythic Familiar mythic class ability (from RGG's Base Mythic Class Abilities). He is a 2nd tier mythic character, so the angel should gain 1 mythic rank with two abilities. I think that a shapechanger familiar able to become an hyppogriff to ride or a parrot on your shoulder would be great, so I'm giving her the change shape (greater polymorph) universal monster ability as a mythic ability. Should it count as one ability or two? It's pretty powerful, but a regular cassisian already has change shape (polymorph), so replacing this with that should count as one... Or not?
There's nothing about the mythic familiar class feature that lets you tack additional abilities onto it. It it treated as having a mythic tier, but does not gain mythic class feature, it's just a source of mythic power now (for those abilities that care if something is from a mythic source). The big boost is that the caster's spells can now originate at the familiar, and even calculate range from the familiar.
That said, change shape is meaty and powerful. At least two abilities worth when added to a cassisian.
Ow. This is a real pain. I was sure I had found the perfect answer to a yearly dilemma, and instead I have to go back homebrewing from scratch. Thank you, however.