Those could be combined, because the ability to bite would be a part of his vessel form rather than a power, in the same way that the ability to cut would be part of the vessel form of a shinki who turned into a sword.

So, "a large, venomous snake" would be the vessel form, and "the power to control mortal snakes" would be the power.

(That said, there aren't too many mortal snakes in, for instance, Tokyo!)