Ran into an issue where a friend of mine couldn't flag the multipet quest if he had Mage Sheep active but could with the Golden Pig. So from this small anecdote, we are assuming that newly added pets cannot proc the quest. Its pretty small, but can be frustrating for players that don't know about it.
We'll give it whirl at the next opportunity. He ended up being able to flag and complete the quest with the Golden Pig pet before the update, so I guess I'd put it out there for anyone to try and replicate what we saw with the "new" pets.
