Honestly I think the 90 minute is ideal for something like this. Predator is roughly 90 minutes of movie if you chop off the opening title sequence(up to first spoken dialogue) and end credits.
If the film had done something similar to what you did in your script, where the Predators are basically setting things up, and doing recon before the actual battle begins, while the humans go through this The Thing esque horror scenario, and we ratchet up the suspense that way, I think it would have worked far better.