Nothing is going to stop you from doing it.  However, its not a Best Practice.  The two different drives are going to have similar, but not the same performance.  So your response times are going to be more varied depending on which drive-type RAID group an LBA has landed on.  If you want my opinion, I'd create two pools, LUNs for each on a different SP.  EMC CLARiiON Best Practices for Performance and Availability FLARE Revision 30.0 in its 'Storage Pool Size' section contains a longer discussion of pool Best Practices.  Best Practices is available on Powerlink.
Should I mix FC400 and FC600 disks in a Pool?  I have a mix of FC400 and FC600 drives in my NS-480 Unified array.  I am planning a migration from traditional RAID groups and MetaLUNs to FAST Pool(s).  All of my FC400 disks are currently allocated, so the FAST pool will initially contain (only) FC600 disks along with SATA and EFD.  When I migrate the MetaLUNs out of the RAID groups into the Pool I will reclaim the FC400 disks.  Would it be better to create a separate pool containing the FC400 disks rather than adding them to the pool that already contains FC600 disks?
FAST Pool and Mixed Disk Capacities