I assume there's rules somewhere explaining that a mascot familiar can't spend a day becoming a team mate of a random monster or passerby, but I can't seem to find those rules. A recurring adversary having a 1 mile radar for the party seems too useful for there not to be a save, and I don't think a player should be able to have their mascot team up with a sleeping dragon for the HP and BAB boost.
It's clear what the intent is, I'm just hoping there's something more explicit. This is mostly to look at the edge case of a mascot allying with a called outsider, but with no restrictions to look at, I'm sort of stumped.
There are no rules on it, but I wouldn't let a mascot treat someone as a team member unless the mascot honestly felt they were a member of the party. So no adversaries, random monsters or passerby---not that you're likely to spend the required entire day with any of those to add them.