You can put SSD on sata2 (interface is backward compatible), but you will suffer serious performance penalty. With sata3 you get 600MB/s transfer speed, with sata2 just 300MB/s (taking into account 8b/10b encoding). And sequencial transfer speed of today's consumer SSDs is approaching even that maximum limit of sata3 interface (i.e. Samsung 840Pro: 520/540 MB/s). If you put SSD onto sata2, you will seriously degrade its performance, maybe to half of its capability. If you want to use SSD for caching, I would not recommend putting SSD on sata2 interface.
I'd suggest to put HDD on sata2 instead (and use sata3-port for SSD). With exception of disk-cache, no common hard-drive has transfer speed higher than 300MB/s. It might suffer a little performance degradation too while using cache, but sure not as much as SSD...