It is possible that Russia will start to use more air bases which each would need a few thousand men for protection and services. I do not think other Russian forces will be introduced as enough forces can be provided by Syria, Iraq, Hezbollah and Iran. There are now enough experienced fighters to help train any increase necessary. Any party who would be unhappy with such an increase can hasten to agree to a cease fire to make it unnecessary.
Russia using more air bases would not only be possible, but be necessary if they really do deploy ground combat troops to take part in the fight directly.
As for relying on the existing forces already fighting on the ground, well I think that may be asking a bit too much of them.
Assad's forces have never been that great, and they have haemorrhaged a lot of men and materials already in this long and bitter war. I have heard that they hardly have any strategic reserves left, and that line troops pretty much never get rotated off to rest and regroup.
That is an unsustainable situation, and if they push the troops too hard and too long, it may backfire and have significant fighting units simply collapse and implode from accumulated attrition and fatigue.
I think fresh Russian and maybe French troops would be able to end this war far quicker and more decisively that relying on Assad's forces and other non-regular outfits already deployed. You want to destroy ISIS after all, not just drive it somewhere else. As such, you want a lightening war, not a slow grind.
I would send in foreign troops to launch a blitzkrieg push right along the Turkish boarder as the opening move, with both a large scale amphibious assault as well as a convectional push from Iraq closing in from both sides and meeting in the middle.
The goal is to cut-off ISIS's supply lines and escape routes as fast as possible to minimise the number of fighters and leaders who could escape as they see the tide shifting decisively against them.
You will also want to secure the board with Iraq (I assume the Israelis can keep ISIS out all by themselves), basically encircle as many ISIS fighters and leadership as possible, and then kill them all.
If possible, I would use foreign troops for all of this, especially Russian and French, as they have a blood debt to pay against ISIS, so would not be tempted by bribes as other forces might be (do not want to repeat America's mistakes at Bora Bora with OBL).
This allows Assad's forces to rest and regroup, and I would use the refreshed and consolidated Syrian government forces as occupation troops to secure, police and hold territory the foreign troops have cleared.
This would be using the assets available to the best of their strengths and abilities.
It should also make it that much more unlikely an insurgency war would ensure and be successful, since the Syrian government forces are locals, so would be far better placed at policing actions then foreigners who don't speak the language and don't know the customs.
There will always been the inevitable screw ups and mistakes in the battles that will cause civilian casualties. Having a force who was not involved in taking a city police it should also reduce a lot of the friction and temptations to seek revenges that often plague the aftermath of battles to take population centres.
