I've found it almost impossible to ally with the hordes. In my most recent game I expanded to Ryazan and had to deal with repeated attacks by the Mongols at Ryazan and the timurid hordes attacking Kiev. Every attempt I made at diplo with them was rejected,hell it was hard enough keeping those damn elephants out of my cities, I only survived due to the fact that I had crushed poland earlier and my Cossack musket spam at all my cities that would be threatened by the hordes,and had alliances with most of the surrounding nations,so I could concentrate at the threat at hand.