You will have a cycle anytime you add new livestock. However, if you add one or 2 fish to an existing tank the cycle will not be noticeable. But if you add 10 fish you will definitely have a significant cycle.Or liverock because it has so much die off usually produces a cycle.
If you have a fish only tank or a reef for that matter as long as you add livestock *gradually* the tank will be able to adjust fairly quickly. Here is a reference about cycling and another reef cycling page Robert