Well, the Borg did stop short of assimilating Icheb's home - because they saw that assimilating bits of it while letting others regrow provided them with more to assimilate. And they have been around for hundreds of millennia, but haven't overrun the galaxy yet. So "holding back" is probably at the very core of their programming...
Replicators tend to suffer from a shortage of FTL starships, but otherwise they expand more aggressively than the Borg, and would probably go exponential as soon as they gained sufficient transportation capacity.
Never heard of the third player. Man of Steel gets silicon implants?
Actually, if you take as a whole of what we know about the Borg, they would only assimilate cultures that have something to offer them, and then only in a concerted, massive effort. Both Hugh and Guinan of suggested that the Borg "don't do anything piecemeal", and probably won't go through the trouble of assimilating a civilization until they've decided that the whole thing is worthy of harvesting.
There are undoubtedly certain cultures that would simply be passed over by the Borg because they're too primitive and offer nothing of value to the collective. Certainly, we've never known the Borg to go and assimilate non-sapient life forms and they don't show a lot of interest in pre-industrial races either. The extent to which they even assimilate PEOPLE is surprisingly inconsistent; they only seem to do this when they need replacements and/or someone to speak for them on a particular subject.