By default, the FSS scheduling class uses the same range of priorities (0 to 59) as the TS and interactive (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.
With the use of processor sets, you can mix TS/IA with FSS in one system as long as all the processes running on each processor set are in either the TS/IA or the FSS scheduling class, so they do not compete for the same CPUs.