I think he originally intended to revive Madara through Nagato...-Just because Tobi witnessed how Edo Tensei works, doesn't mean he can perform it.
-You don't know for sure he can attain enough genetic sample of Madara.
-Resurrecting someone with Rinnegan has a sort of "expiration date". If Tobi could've resurrected anyone from any time all on his own, he wouldn't have much need of Kabuto, now would he?