When was is stated that Ems was required for rinnegan i thought it was just the combination of uchiha and senju?
Kishi hasn't stated ANYTHING that is "required" for the rinnegan. These are the facts we know:
1) Madara is the only person, after the great sage, to unlock Rinnegan
2) Madara unlocked rinnegan before he died (LOOOONG after he was an old man)
3) Madara happened to have Hashirama's cells embedded in his chest at the time.
4) Madara happened to have EMS
5) Madara was an unusually powerful Uchiha, and an unusually skilled sharingan user.
6) Madara was unusually well-read about the sage, the rinnegan, the juubi and other apocryphal topics.
Point number 1 suggests that there isn't a simple formula to do it, and Point number 2 suggests that even when you have the correct ingredients it takes a very very long time to make it work. Even if Sasuke got Senju DNA embedded into himself, it may take him years and years for the Rinnegan to awaken.
The rest of the points (3-6) are all things that are unique to Madara or very uncommon about him, so it's hard to know which of these were important in awakening the Rinnegan or not. Madara had Hashirama's DNA specifically, not just random Senju DNA. It would have been much easier to Madara to kidnap and kill a Senju child instead of fighting Hashirama to the death. But that's not what he did. He got Hashirama's DNA specifically, which suggests there is something uniquely special about Hashirama.
Kishi never explicitly told us the formula. Maybe it's something easy:
Senju DNA + Uchiha DNA + Time = Rinnegan.
Maybe it's something more complicated:
Senju DNA + Wood Style + Uchiha DNA + EMS + Time = Rinnegan
Since Madara had EMS and he was uniquely well-educated about the sage of six paths, maybe there's some other secret or incantation or technique that needs to be used before the rinnegan is unlocked?