The only issues using smaller blocks is it restricts the size of line you can use. (series 40 would be like 8mm)
I personally think 8:1 is as much as anyone would ever use on a mozzi (read small girls)!!
The norm is 6:1 with 10 or 12mm line keeping in mind larger diameter lines grip better in the ratchet but don't run as nicely and you find that the diameter differs greatly depending on brands and quality