There is a difference between a "cycle" and the "intial cycle". Any tank, even a mature tank can have a "mini cycle" when something goes out of whack, but it's not common from anything I've ever seen, for you to start reading ammonia and nitrite again (especially on a mature tank). Usually in a case like this you will see an algae bloom or something from increased nitrate levels, but hardly ever do ammonia and nitrite levels re-appear. All tanks need to find their balance and it took my tank a full year to do this. My
intial cycle was completed about 2 months in. The initial cycle being ammonia rising and then as it is converted into nitrite by one type of aerobic bacteria, it will begin to fall off as nitrites levels shoot up. Once this occurs, another aerobic bateria steps in and starts to break down nitrite into less toxic nitrate. Once you have reached this point where both ammonia and nitrite levels had risen and then fell off to a zero reading and remains there for a few days, then your know your initial cycle is complete. This is the point at which you can start to introduce fish into the tank slowly. From here, you are left with nitrates and it's through your tank maturing and building up the necessary beneficial anaerobic bacterial colonies that these nitrates can be worked off through biological filtration. Once you get these numbers down to zero, this is when you know your tank has found it's balance and can be considered completely cycled. With that said, things change within a tank from time to time as new fish, corals, inverts etc are added and in some cases you may have things die. During these times, the tank will have to adjust to either the increased bioload or decreased bioload and you may experience a mini cycle, but hardly ever will you see ammonia or nitrite re-surface on a mature tank. A newly setup tank is a different story as they don't have the stability a mature tak does.
Anyway, that's my personal thoughts on it. Obviously the tank isn't balanced or matured yet if nitrites are reading so if you want to add in more liverock this would be the time to do it IMO. Waiting till the tank has balanced and adding it afterwards will basically be like you are starting all over again if you experience a lot of die-off so now would be the time. I'd get all the rock you want to put in there in there, let the thing cycle out all together one time and be done with it. Either that or you could cure the liverock in a seperate container and once it is completely cured you could toss it in the tank at any time as there would be nothing left to die-off in the tank so no issues to worry about. Either that or add base rock at any time.
As mentioned, just my personal opinion and not "gospel". Many ways to do things so go with what works for you.