Thing is, though, Shop pets are NOT “Retail”. They are BattleNet wide.
I agree that “cross-collection shenanigans” are unlikely, but Shop pets are not really cross-collection.
I didn’t have any Shop pets back in Mists, but I did have Murkalot from the Virtual Blizzcon, and it was one of my favorite battlers when I was first levelling, so externally acquired BattleNet pets did battle.
It is an interesting question.
Wowhead Mists Classic lists Lil’ Rag as a Battle Pet, with stats and abilities
but Lil’Rag was available in Mists. Wowhead Mists Classic also lists Murkalot as a Battle Pet.
But it does not list later pets like Daisy and Shadow.
So my guess is that only pets that were available to battle in original Mists will be available in MoP Classic.
Which seems appropriate. 