it's not prohibited to use those drives, just need to be careful what type of workload you place there:  Look for section that talks about vault drives  [URL]
I heard statement that it is prohibited to use free space on first 5 disks in FC shelf, where are located Clariion and Celerra files,  that this is against guarantee agreement and if you use it you not only lose gurantee, but also may have problems with perfomance of celerra   I am new to Celerra, but I am sucseffuly used these disks on Clariion CX3-10c  are this statements true or not? 
Using free space on first five FC disks on Celerra NS-120