The criterion to define a specific PFC are fully explicited in the core network (ggsn, hlr)
The most logical way is to associate it with a specific service (a specific APN), but I guess you could also associate it with specifi IMSI’s.
MS with GBR will totally crush other MS. You will increase your congestion rate, you will decrease the throughput for the nomal MS.
But you will ensure that throughput for the special MS is always available.
regards
pix