I don’t use sleds a lot but when they are part of the plan for the “newbies” I will put them in fairly often over the course of say 3 weeks using quite light weights (10lbs, 20lbs, 25lbs so about 10% of BW or less) and then observe how much the ground contacts change and what the natural urge is regarding how upright they run at what point, 20m max. This will be in a training phase, GPP, where they can afford to lose some training time because of familiarity. Later I will re-introduce after a couple of 6 week cycles “off” and then have used “sparingly” except for the two weeks immediately prior to the goal competition.
Once “everything” has been introduced the pattern I use is resisted runs first, never longer than 30m, body weight runs next anywhere from about a third less distance, 20m, to double, 60m, and then assisted if that is possible (logistical issues).
Another occasionally used resisted run series is using the wind and those will be again 30m or less into, followed by an immediate turn-a-round lasting, at most, 70m (the 100m total is the thing I am looking for at the longest) depending on the day, this can definitely get into quite the over speed, so much so care has to be taken where on the track as control has been lost trying to negotiate curves.
I have made more use of resisted partnered runs, only partially because of resistance. I have found it more useful as a teaching tool, a little bit for ground contact and foot speed but mainly for trying to teach about body position and angles.