Tobi is far too knowledgeable to be anyone other than Izuna. He knows things about Madara and Izuna as if he were there personally. He has to be an Uchiha because it's the only explanation for the way he can utilize the Uchiha blood line limits (sharingan, rinnegan). Besides, it just makes sense that he's Izuna Uchiha. No one else fits the timeline, motive, or even the personality traits.
Thanks.
Anyone can make a contract with the toads. In fact, anyone can make a contract with any of the animals. The reason why Naruto made a contract with the toads, was because his master did, the same reason that Minato did.