Pieman... Dude every thread I have read with your comments have had you begging for feedback. Make a cup of Tea, sit back get back to modelling and relax. People will reply to you and contribute, but you've got to stop begging for attention. Seriously mate it doesn't look good.
Remember you're super cool, you dont need the attention of us mere mortals!
Now, to the point of the conversion. I'm a little confused as to which model your using as the basis of the conversion. Are you using the plastic Tank commander that comes with the Devil fish sprue or the resin Forgeworld commander? Depending on which one you are using there will be different stylistic things to consider. For example the FW model is slightly more heavily armoured and looks more like a combat ready firewarrior, whilst the plastic one is a bit more combat fatigues and looks uncomfortable outside of a tank (he would however make a good basis for a pathfinder as they typically use less armour than their firewarrior cousins).
I'm not sure what side the spare arm is since you havent specified left or right. This will matter since there are certain limbs on the firewarrior sprue that are left/right specific (for example the little tau scanner arm). One option is using the binoculars, however that will make him look a little too tank commander in my view and you want him to be more bad a** Shas ui.
You could use the scanner arm to make him look like he is issuing orders to squad mates or a drone, or you could give him the hand grenade arm to make it look like he's priming and throwing an EMP grenade into combat. Alternatively you could take the empty Tau arm from any of the rifle/carbine holding poses on the standard sprue and reposition it to look like hes holding his helmet under his arm. Finally you could reposition the empty arm up and around to have him look like he is reloading the pulse rifle that he's propt up on his thigh. Or he could be holding his honour blade in the empty hand to display his authority and the bond that he has with his squad mates.
The possibilities are endless, and if you get the chance why not post a photo if you can? I'd be interested to see what you go for and how the final model works.