The SR has a fin designed especially for it. You could get away with the 9" HP fin and certainly a 9" Greenough 4A would work well, but if you want to experience the ride as it was intended, I'd recommend calling the shop and picking up the SR fin. The 7.5" is too small unless the board is a 2+1 and you want to ride it that way. If it is a 2+1 I still recommend picking up the 9" so you can enjoy it as a single fin as it is really superb.

J