and I want to avoid this "Everyone is hostile at the beginning" thing I've been hearing about.How do I avoid this?

This again? When you coross the wooden bridge, don't shoot the guy on the rock. There's a friendly person there.

In fact, don't shoot at all. Just hide behind a rock when the fighting starts. There are only 2 people you will need to kill. And they are just before you get to the bridge. But like MtnDewFanatic said, don't shoot the guy on the otherside of the bridge on the rock to the left.

I almost shot the guy on the rock. He was zeroed in in VATS but I noticed that he was in green, not red. I was shooting as many bad guys as I could trying to keep the caravan alive and almost got too trigger happy.

Just remember, you cannot save anyone in the caravan, only shoot those that shoot you first, and don't shoot the non-hostile dude in the beginning unless you want everyone to hate you. I might try it just once for the ongoing combat.