you can not cause a fire a computer fan will only work in one polarity, if you reverse it it acts like an open circuit. also one might have a white stripe running down it or it will have little white dots. but you will quickly realize whether or not you have it in the right polarity.
twist them and tape them it should be fine. If you are really concerned about fire you can do a couple of things like put the transformer in a pie tin with nothing over it, a baking dish would also work. dont worry heres the rub if anything would catch on fire it would be the fan now while fire will conduct electricity there isnt that much stuff to burn there so most likely it would burn itself out. I woulent worry about this. What i would worry about is the smell you are obviously in school, most likely living at home. you do realize that growing weed smells about 10x stronger dried cured weed.