I would also expect they will be used as lead in fighter trainers for the PAK FA pilots
I had this thought, but I wouldn't be suprised if they followed the USAF model instead. We (and this admittedly works for us given the number of airframes we've got to mess with) take a pilot, put him/her into training. After basic flight school, you're going either into the fighter/bomber track, which means off to T-38s, or the heavy track, which means off to I forget what. Once you finish advanced flight training, your first assignment is to a training unit. For the F-22, for example, you go to Tyndall AFB. There your sole job is to learn how to fly and fight the Raptor. After that, it's off to an oeprational unit. If you change airframes, you merely go to that jet's training unit. My dad did this when he went from F-4s to A-10s; he went for a few months to the A-10 training unit at Davis-Monthan before ending up at RAF Alconbury.
If you don't have the airframes to do that, the simplest solution is what the F-117s did. They had three squadrons, one of which was the training squadron. You got assigned there first to learn the jet, then off to one of the two operational squadrons.
I haven't paid much attention to the ORBAT of the RusAF recently, but I think they do something similar to the first, except they consolidate training for multiple types at one location.
And I think the MiG-29 was compatible with the R-27T and R-27P, they just weren't employed often.