SAA is Syrian Arab Army. SDF is a mix of Arabs, Asyrian and Kurds, primarily Kurdish fighters.So many sides in this conflict, it's difficult to keep track of the acronyms!
The SDF are the Kurds, and the SAA is the Syrian Army? Do you think we'll see fighting between Russian-backed Syrian Army and the Turks? I'm thinking Russia must have some agreement in place with the Turks or else Turkey would not have made its move. The fact that we haven't seen any condemnation of this from Russia is a strong indicator of this.
Russia did not condemn but did voice disapproval, even the Syrian government's voice was very mild "defend territory with all legal means" instead of "drive out invaders". So yes, there is some understanding (of Turkish move) among every party except the Kurds. That is the common ground I said.
The Turks may want to insert their proxy in the post civil war Syrian government, so they are supporting the FSF remnant and trying to replace Kurds with FSA at the border. But forcefully doing so in the future by directly fighting SAA would be suicidal.
By invading Syria, it has already lost support from the west. By fighting SAA on Syrian soil, Turkey will become enemy of the world. Yesterday, China has called Turkey to pull out without condemnation in the same line of Russia, the last of the UNSC P5 to make a statement. I see that as the line drawn by all powers.