Hey, Elliot!
While elementals of counter elements are usually uncomfortable around each other, this feeling can be overcome if they truly do try to get along (or at least tolerate each other). This is more common with older elementals.
Being uncomfortable around a counter element comes naturally to elementals, and this makes getting along with an elemental of a counter element a little more difficult for younger, less experienced elementals. I wouldn't say it's impossible, but I think it would definitely take a lot of discipline and self-control.
As for the Harbingers, I think that at the end of the day, the decision making ultimately depends on their ideals as opposed to how they feel around each other. The discomfort counter elementals have around each other does not necessarily equate to a disapproval of everything the other does. I like to think that if two elementals share the same views, despite being counters, they would admit it. They may be reluctant to, or go about admitting it in a rude way (e.g., Harbinger Typh saying, "While I hate to admit it, I think Harbinger Demin may actually be right.") but it would still happen.
That one is a bit tricky to answer, however, as it truly depends on the driver and how they choose to play their Harbinger character. I think that, if the Harbinger is unable to put their discomfort aside or has an intense hatred for their counter, rather than mere dislike, then when an agreement can not happen, the final decision is ultimately made by the Ice Harbinger, since (s)he is the leader.
Other administrators and members can feel free to add to what I've said, or even contradict me. I do hope that I've managed to somehow answer your questions, though, or at least help in some way, haha. If you have any more, let me know. ^^