"I agree that the plate armour for the Gondorians was a lot more feeble in numerous instances than plate would actually be."
I certainly think you're right. There's one part in the extended DVD where a soldier gets shot by an arrow from a short composite bow, and it goes straight through the most heavily sloped part of his breastplate.
As for the orcs swarming, in the books themselves this is how Tolkien describes the fighting; the tall, heavily armoured Gondorians would only be overcome by a small crowd of the smaller, weaker orcs.

