Personally, I find hover-handers hysterical. I mean, not in a bad way, but most cosplayers will be chill if you put your arm around their shoulders, but...!
If you say "can I put my arm around you", by that you mean you want an arm around a waist, it's important to make it obvious or do it slowly, so you don't startle the cosplayer. (Mostly for girls, but I've seen some guys freak out too.)
In general, most people love getting their pictures taken, whether it be with someone or by themselves.
You don't seem like the type to do this, but if the cosplayer is *ahem* gifted, make sure that your camera is pointed at the right angle, as to not give them the wrong idea. >_<