Obviously you can't have a shifter Mage under the priority system, but is there anything in the rules that disallows a Shifter being an Adept?

You can be a shifter adept but your powers don't work in animal form

And you can be a full shifter mage under the priority system, the racial priority just goes down a slot.

I thought you had to have Race A to be a Shifter?

It mentions in the same paragraph that if you also want to be a full mage, then race becomes priority B, and magic becomes priority A. It is the exception.

What book is Shifter actually in? I've been going by NSRGC

Shadowrun Companion.