I don't flee with Beastmen chaff because there's a good chance they're not going to rally. I'll make an exception if it's the chaff that is running around inside the generals LD OR they're ungor raiders with a musician (this is the reason I give them horns, LD 7(!) on those rally tests)
In 2500 points my chaff typically consists of: 3-4 units of 5 ungor raiders 2-3 units of dogs 3 solo razorgors 1-2 units of 5 harpies
After I've won the chaff war or the enemy chaff is preoccupied and out of the way I usually still have plenty of chaff to gum up the workings of the enemy for a few turns while my Gors and Bestigors clean up the crucial combats.
My favorite character assassins are razorgors. for 55 points you get 4 WS3 S6 attacks against a single target. If you take a unit of 2 and place them 1x2 then you get 7 S6 attacks against a single target. More than enough to statistically kill a pesky mage/warrior priest/bsb.
Of course mine tend to miss with 5-6 of those attacks but that doesn't stop the theory from being sound, I just have bad luck with that tactic