I'm a-okay with having people in normal cloths wanting to have a picture with me. I mean, I'm guessing they want a memory with their favorite character or something and I'm happy to oblige.
As for putting an arm around the shoulder, for this I have to STRESS that you need to ask the cosplayer before you touch them at all. Everyone has different comfort levels. While one cosplayer might be fine with it another won't. So it is better to just ask to be safe.
For wording it you could say, "May I put my arm around your shoulder if you are okay with it?" Just be polite and courteous.