To me, it sounds a lot like playing. I think you would definitely know fighting when you saw it. With the biting on the back of the neck, like you said, that's Barnaby's play style. They'll probably get a feel for each other and you'll hear less of that. However, it'll always happen. Sometimes cats get carried away during play and hurt each other. (Especially male cats, but it even happens with my two girls.) It's nothing to worry about really.