I agree with most of that but we know Rebel factions have fought against ISIS, one of these US jihadists who joined ISIS was killed by one of the Rebel Factions.
Obviously, I don’t know this for myself but plenty has been written on Assad trying to weaken some rebel groups by bombing them and that actually helps ISIS. These are just two articles of many, including what some Syrians say, that say this.
http://www.businessinsider.com/why-assads-air-force-is-now-a-force-multiplier-for-isis-2015-6
http://lists.csbs.utah.edu/pipermail/marxism/2015-June/262209.html
Lt. Col. Oliver North likewise, I know has favored certain Rebel groups as well: http://blog.godreports.com/2015/08/oliver-norths-five-point-plan-to-defeat-isis/ It is not clear to me if he is talking about Syria or Iraq. We don’t hear as much about “other rebel groups” in Iraq.
Putin also said Assad is ready to share power, that most likely means there is a viable opposition. I doubt if Putin inferred sharing power with the ISIS devils.