Thanks to all for the replies; you decreased our anxiety, and we settled in and watched the cats but didn't separate them. They've calmed down, and definitely were just figuring each other out....two happy kitties!
We just introduced a new kitten (2 months) to a recently adopted cat (1 year old; we've had him for two months). They're both playful cats on their own. We introduced the cats as has been suggested (kept the kitten in a smaller area with its own litter box and food), contact done bit by bit...
