Moment the emergence of a new population in apartment the most complex. Cats more intolerant in respect their enemies, so option “the first cat, and the second — dog” enough dangerous. At no time ignore both the first time!
In the event of a conflict it is reasonable to limit the movement of the dog around the house for the first time, if you have no ability to look after them, and very important — do not allow the cat to get to where you left the dog. Do not punish neither the one nor the other for the aggression to the second “tenant”, it will not teach them! Better to promote friendliness.
For example, a dog approached, sniffed the kitten and walked away quietly — treat it with something. The cat stopped hissing and strain when the appearance of a dog — the same, for this she deserve a reward.