I'm not following you on how the Guardians appearing in an Avengers movie makes a difference. If you understand it was not for all characters then what would be different? Comic book characters and their respective families interact all the time.
Take for instance Big Hero 6:
http://marvel.wikia.com/wiki/Big_Hero_Six_(Earth-616) they have interacted with Spider-Man and the X-Men. This happens because they are Marvel properties and exist within the same universe. Should that mean because of comic interactions that Universal has theme park rights to Baymax?