I once brewed a batch and had to put it in the a 6 gal carboy because all my buckets were occupied. For some reason, perhaps our boil wasn't very aggresive, I only got down to 5.5 gals instead of 5. Two days later the airlock, plug, and krausen were all over the floor.
Be sure to leave yourself plenty of headspace and you'll be fine. I don't recommend using the same yeast for two different style batches, but if your happy with the flavor that's okay. As for the contamination, when I had my problem my beer turned out really tart. Good luck.