Testing results: Sorry to disappoint, but it appears that creatures with this addition actually do light things on fire. Other than that, though, they appear to work as Deon describes. Couldn't test breeding capability in the arena, so no data on that.
Test 1: Modded GCS vs modded GCS. They were immobile, as expected, and spammed webbing at each other when sufficiently close together. Maximum firing range seemed to be 5 tiles (4 tiles empty space between creatures), but that might just be normal for GCS. Observed that webbing landing on a square with GCS in it would burn, producing copious amounts of smoke.
Test 2: Modded GCS vs Giant. Spawned creatures 10 tiles from each other. Giant moved towards GCS, with GCS spamming webs at Giant when it moved within range. Giant stopped 2 tiles away from GCS and backed off, as though unable to reach target. No further interaction.
Test 2a: Took control of Giant and moved to attack GCS. Moving into a tile adjacent to GCS required an alt+move. GCS attacked and immobilized Giant with webs and poison, and many many pages of the Giant melting and then burning ensued before Giant's death.
Test 3: Modded Giant vs. modded Giant. When spawned away from each other, neither moved to attack. When spawned adjacent to each other, each attacked and dodged as normal. However, when dodging resulted in space between each creature, both ceased attacks immediately. No further interaction.
(I also tested GCS by lighting them on fire with magma and dragonfire, and they behaved similarly to the modded versions with the exception of burning to death rather quickly. Don't feel like writing those tests up; they were messy.)
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To sum up, I'm pretty sure this behavior is due to the fire avoidance code. And unfortunately, even though the material isn't actually used in the creature, the fixed temp is. Why the creature doesn't light itself on fire I have no clue. This also means that, if used in fort mode, dwarves would likely refuse to move/chain the creature due to temperature avoidance.
I feel like there's a bug report in here somewhere, but damned if I know where.