Double bonuses from a stat stack so long as they are different bonuses. It is entirely legit.
For example, the second level of Deepwarden allows the dwarf to use his Con mod in place of his Dex mod for AC. It's still treated like the Dex mod. A level in Fist of the Forest also grants Con to AC, but in this case it's just like a Monk's Wis to AC. They are different bonuses, and thus they stack. This exact combo is what gives Fistbeard Beardfist such good AC.
Another example I guess: A Monk/Sorcerer with the Ascetic Mage feat applies his Charisma bonus to AC in place of Wis. Several incorporeal templates add Charisma to AC as a Deflection bonus. Again, these stack because they are different bonuses even though they're from the same stat.
And looking at the Primeval PrC, I'd agree it could get pretty insane. There is the whole max 8HD limit though, so for the purposes of Primeval Form I'd recommend a Dire Wolverine.