All races are fine, but Dryads and Treants are their own thing. While a Treant is an animated tree, a Dryad is a Tree Faerie. Though they are related, and they hang around each other, they definitely don't branch (haha) into each other racially. If you're looking to be a big damn tree, you might try a sort of progression with the race name. 

Treant should probably be your starting racial levels, and my best suggestion is to come up with a progression for a more prestigious sounding tree creature if that's what your desired final result is.

You could make it on the nose and obvious like... Treant -> Elder Treant. I don't know if you could call a Treant an elemental, though. Jury's out on that one.

You could also make it more abstract like... Triffid -> Thornfiend -> Jilted Lover. Not actually that, of course. That's not the creature you're making, but it's an example of the formula.

Even Momonga uses a more abstract progression for his racial levels, as his first are Skeleton Mage (which is probably based off a basic creature to fight), and then advances into Elder Lich and then Overlord.

Your cohorts should [i]definitely[/i] include at least one faerie of some type, especially if whoever designed the NPC got strangely and hugely into European folklore. 

Anyway, other than the mentioned oddity with your racial progression, this idea sounds solid to me! Was he a roleplayer or just a normal player?