I would think as long as they are not biting ,scratching or attacking the kitten they should be just fine. Hissing is just the others way of telling the little fuzz ball to stay out of his space. He obviously feels like he is big cat on campus. As long as fur, fangs and claws are not flying I imagine they will be buds in just a short time, curled up together licking and cleaning each other. We definitely need pictures though!

Is the kitten a boy or a girl?
How about Slater or Slate, as in a color of grey if a boy. And if it is a girl, Pearl.