I’d keep sled work early in the speed session as a stimulus for the rest of the speed work and I kept it short generally because it was most likely to avoid hamstring tendon stress because the runner will still be in a forward acceleration position.
I wouldn’t use a sled for FEFs because of yanking on a slack line during the re-acceleration and I also wouldn’t use it for fly-ins because you’re fully upright going in- both hard on the hamstring attachments.