Any reason they couldn't be aligned to both demonic and magical? The character gets the gift from one magical parent and the demonic blood/power from the other parent.

Thing is, we have a With of Thessaly, and we need a suitable Major Virtue. It can't be Hermetic, she already has one of those, namely Chthonic Magic
Well, summoning? The magical version?

Any reason they couldn't be aligned to both demonic and magical? The character gets the gift from one magical parent and the demonic blood/power from the other parent.
Having power from 2 realms that way is possible. You can be a magus with Incantation, for exemple.
But demonic blood clearly makes you non-human. This is a neat difference. It's like if you were a nephilim: You may look like a human, but you ain't one.
Summoning the Magical Type is the Way we went.
(We have one of these too.)