Ethics aside, can I drop in on/sneak into college cles?

I would definitely recommend that you be up front with the instructor if you're thinking about unofficially "auditing" - the actual college-y word for what you want to do - cles that are even the size of your friend's psychology cl, let alone smaller ones. If you want to participate actively, I would talk to the instructor first regardless of cl size. I worked with university faculty for over 20 years and I can tell that there is, if anything, even more variation in personality and attitude and behavior among them than in the non-teaching public. 9 out of 10 would have no problem with what you're suggesting; you just don't want to cross that 10th one's path...