I hate to disagree with you Pawain, but nightmares can only get one extra special. They have magery and dragon breath. And there is no way to “overwrite” dragon breath. I know this because I tried to do just what you suggested and lost the ability to add anything after I picked goo.
I have 2 suggestions, either add AI to the nightmare and be happy. Or if you really have your heart set on having both goo and AI, then go with another pet, fire steed, unicorn (if toon is female) or ki-rin (if toon is male).
I personally have a fire steed with goo, AI and chiv.