It will only take one good argument between those two and the territory will be chosen. I would just let them out to get to know each other. the kitten you already have might just take your place and baby the new kitten. You have to let them get to know each other on their own terms. You protecting the new one is just going to make the older one more jealous as if you like the new kitten better. I have 7 and each time I have put a new one in the house, I just let them all be together and they calm down faster that way. I do not show any favoritism over one cat. good luck, they will be fine.