12v question regarding coolant

OK, I know that mixing G12 coolant with regular green coolant ends up turning it to a jelly of some sort, and it's obviously very bad for the engine. However, what if the G12 was drained fully before green coolant was put in? What might arise because of this? What exactly is different about the G12 that we are required to use it. Is the regular green coolant just not good enough, or is there some other property that keeps us from using it?

I ask this because I went to pick up a 96 12v on Sunday, but didn't bring it home as the water pump was leaking through the weephole, and I wasn't about to drive it 3 hours like that. I may still get it and haul it home, but it's leaking green coolant. If the G12 was fully flushed beforehand, what kind of problems am I looking at, if any?