I'd love to replicate both, and I would suggest that there are a number of realms where 600 men might be their whole army, especially on a long-distance campaign. Hell, I remember that when Arcaea was fighting Zonasa on FEI, we only got 1000 men into our army once, every other time was fewer. Now, granted, we were using mostly high-end troops, but still.
600 men is a small army. I don't mean to say that it's a bad army, just that it's on the smallish side. It's normal for small realm to have small armies; with such a proposal a small realm that succeed in luring its bigger enemy to a mountain region would have a better chance.
Your exemple of Arcaea is also a good example. That was a small (for Arcaea) specialized force of high-end troops. The proposal is exactly aimed at giving more tactical advantage to such armies instead of the big blobs.
As for Thermopylae, if it really happened with numbers even close to what is described in the legends (and even that was 1400 men, 300 only counts the Spartans), it was a freakish battle, and it only worked because they were only fighting with lances and shields and somehow refused to fight at night, therefore giving the Spartans time to recuperate. In medieval times, a catapult would have been used and the pass cleared in a matter of minutes.
