The new rock will either be good enough that you won't need any new bacteria, since it will provide more than enough of it's own, OR it will be crappy rock that will spend a few weeks rotting in your tank at a rate you can't accommodate with additions of bacteria. This is why adding all of your rock at one time is a good idea if you can afford it; you get to avoid this hassle.
Your better bet would be to either make SURE you're getting good rock that won't rot & cycle, or to set up a curing tank to let it cycle outside of your main tank.
Downsides to adding needless bacteria- oxygen depletion by excess bacteria, dieoff of excess bacteria, starvation/suffocation of liverock bacteria, waste of money.
Ordinarily, adding bacteria is a good and very useful thing. When new rock is concerned, though, it isn't. Your filters will cycle fast, of course, but at the severe expense of your liverock bacteria at THE very time you can't afford that.
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