Noo sorry but it's not reasonable. Chuunin or genin, still sounds ridiculous. Madara and Hashirama have been the only ones ever capable of controlling the Kyuubi. Obito isn't on the same level just because Madara taught him a few years. Get real.
Hashirama couldn't control Kyuubi, he captured. Big difference there.
Also it is not unreasonable, Madara is the top Uchiha to exist, he knows more about the Sharingan than anyone else..its not unbelievable that Obito while under Madara could progress so quickly.
Imagine if Sasuke had actually been trained by Itachi instead of Kakashi, Kakashi is great but hes no Uchiha, he can't teach things hes incapable of doing.
Madara is in a league of his own and considering his age and the fact he was the first person to unlock EMS, and second person to possess Rinnegan its not unfathomable that he could teach a teenager how to use the Sharingan to control Kyuubi