@PsychicHobo hits the nail on the head. We have a number of great units in our FA slot, and no good way of getting them into a list (without resorting to CWE, but thats a slippery slope).
Yes all the beasts in the Beastmaster squad get SfD
As for the Razorwing flocks, they may not be the best choice. Although they have some decent damage output (against 2/3+ enemies), they will still suffer against the ID caused by S6 fire (aka Scatterbikes).
When they actually get to make use of their 3 wounds, they are great at 20 pts. When they get insta-gibbed by a single melta gun, not so much.
For beastpacks, generally you want to go with the Khmare. They are one of our best CC units (pointwise), and reasonably tough. I would look at them to make up the majority of your beastpack, and maybe try to hide 1-2 Razorwing flocks to try to use the Rending and multiple wounds (while keeping that sweet T4 majority).
-This from a guy trying to make the beastpack work for the last year (unsuccessfully)