In an Earth/Thermal build, I would ALWAYS plan on the 2 +Resist IOs for pets placed in Animate Stone. This allows him to have over 80% resists vs Smashing, and capped vs everything else, which means he can then tank AVs for a party. It's too great an ability to skip on it for just the cost of two enhancement slots, and slightly underslotting the pet. And, with all the -Def from Earth, you don't really need much Accuracy slotting in Animate Stone.
In fact, if you 4-slot Expedient Reinforcement using one of the Resist IOs, I think that you can use one more slot to get over 90% Damage, so the pet ends up with the damage that you need, 20% or so Accuracy (with the -Def that's good enough), and +20% to all Resists, while the set nets you some great IO bonuses (6.25% Recharge and +3% Damage).