Assad doesn’t fight ISIS, Russia doesn’t fight ISIS. Sad.
I wouldn't be surprised in the least if ISIS was actually a Russian invention, created for the sole purpose of given them 'reason' to invade other countries. They create instability, then they 'come to the rescue'. Sort of like the power-seeking firefighter who deliberately sets fires in order to make people think he's this 'wonderful' hero for being first on the scene to put it out.
Knowing Russia, their history, and how they operate, you really can't put anything past them.
From the perspective of Assad and Russia, they have to consolidate power in Syria first, then they can attack ISIS.