You can definitely just replace the body definition with whatever you need it to be. The way the iron man's tissues are defined it will just apply them to every body part you add. However, this means even broken teeth will emit steam.
To replace the gas with steam, you can outright delete the [USE_MATERIAL_TEMPLATE:GAS:CREATURE_EXTRACT_TEMPLATE] and accompanying syndrome. Then change the tissue definition for the gas here:
[TISSUE:GAS]
[TISSUE_NAME:gas:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:GAS]
to:
[TISSUE:STEAM]
[TISSUE_NAME:steam:NP]
[TISSUE_MATERIAL:WATER:NONE] - because water is actually a valid material
And remember to change the tissue placement string to:
[TISSUE_LAYER_UNDER:BY_CATEGORY:ALL:STEAM]
Although, that gives some weird results with the game saying, once the creature has run out of steam, that all parts of its body are listed "condensed" in the creature description and all body parts will be colored brown in the wounds screen. This isn't the case for the standard iron man, and I have no idea what being condensed means to the game. I've never encountered that before.
I would try making the steam blood instead, I guess. They would then die when they've bled away all their steam (In testing, with the iron tissue having VASCULAR:1 this doesn't happen even when lopping everything off one at a time, so it's not a real threat to them at that level, but they do bleed steam from the wounds like they would if they had the steam as a tissue layer.) and it would also avoid this "condensed" issue.
You can do that by removing the steam tissue and material definitions and replacing it with:
[BLOOD:WATER:GAS]
and [VASCULAR:x] in the iron's tissue definition.
You won't have to change their body temperature (although you could, so that they're hot to the touch.) and I don't think they'll actually have any problems in extremes of temperature as iron doesn't have a cold damage point. They could melt, though. But at 12768 urists they'd still be magma-safe.
Now, if you don't want the teeth to bleed when cut, you'd have to have two different iron tissues, one that bleeds and one that doesn't, using the non-bleeding tissue on the teeth. You can even give them internal organs.
Also, have you considered making them out of brass? They'd be a little softer, but still reasonably tough compared to everything else.