In general, I would look at these two factors when looking to include cards not of your tribe's creature type: Is this creature so strong that it, on its own, is generally stronger than my tribal creatures when they are receiving bonuses and/or empowering my other cards
It has synergies with humans too, especially ones with good ETB effects
One of the wild things about Doctor Who is that its been around for 60 years and theres so much to cover
Pull rate from Rare Packs: 2.56% The estimated pull rate for Vaporeon (Wisdom of Sea and Sky) from a Rare Pack is approximately 2.56%, meaning you might need to open around 40 Rare Packs to obtain it
Despite the type being around in the game for so long, there aren't as many Bat creatures as you would expect
A Moonshaker Cavalry will eventually show up, ending the game in short order