Have confirmed with our NS-480 that anything less than 8 drives will not work for Fast Cache when using 200G SSDs.  We had 7 unallocated 200G SSDs and Unisphere just threw error that there is no supported SSD drive configuration.

Consensus seems to be that smaller drives are better for Fast Cache anyway and will support more configurations:

As in Appendix A of doc link below:

Hot Spares are optional but recommended.

Note that the specification sheet for the ns-480 states that only 73G, 200G, and 400G SSD/EFD drives are supported so that should be verified as well before going out and buying 100G SSD.

Cannot confirm those statements yet, as I do not have the smaller SSD drives, but plan is to move the 200G drives to pool and allocate 4 new 73s to the Fast Cache.

According to friends, almost any combination is possible although dependent upon metadata structures and size limits.

See http://www.emc.com/collateral/software/white-papers/h8046-clariion-celerra-unified-fast-cache-wp.pdf for more details on Fast Cache

Make special note of the section "Best practices when you have limited number of Flash drives" on page 17, while also making note that in the simplest of terms, Fast Cache is great for bursty traffic while it's not so great for sequential traffic.