The normal range of fair share scheduler class priorities is -60 to 60, which get mapped by the scheduler into dispatch priorities in the same range (0 to 59) as the TS and IA scheduling classes. All LWPs in a process must run in the same scheduling class. The FSS class schedules individual LWPs, not whole processes. Thus, a mix of processes in the FSS and TS/IA classes could result in unexpected scheduling behavior in both cases.
The TS/IA or the FSS scheduling class processes do not compete for the same CPUs. Processor sets enable mixing TS/IA with FSS in a system. However, all processes in each processor set must be in either the TS/IA or the FSS scheduling class.