you are right, the green plug is just for the auto lights and crap. Leave it unplugged and you'll be fine. I used my car to check an auto trans cluster and it worked fine.
BTW, DO NOT use
http://patriot.net/~merritt/gauge.html as indication of what wires do what on a 92-95 civic cluster. I've found four wires that are wrong on there all ready and I've only checked six. (pulled that link for the sticky)