The rules say you can trade fire extinguishers. Are there any conditions or limitations to this - can you trade empty and partially-complete fire extinguishers?
Yep, if the person trading it is silly enough to part with it and the elements on it.
Is there no limit to the number of extinguishers in varying states of completion you can hold at one time?
Nope. Go for it, be the Extinguisher King.