The AI behaviours and build-priorities are all set using the same algorithms. If they've (all) run out of space to settle (or they can't build Settlers), they will build military units instead, and once they consider themselves stronger than an opponent, they'll usually find some excuse (or none at all) to declare.
So e.g. if you've not been matching their military production, and they're now all stronger than you, then they'll all pick you as the weakest potential target.
Yes. Open a new .biq, go to the "File" pull-down menu, and choose "Import..." After you've clicked through the warning, you'll get a pop-up box called "Import BIC data".
Click the check-boxes for "Rules" and "Players", then select the original .biq file that you want to import those details from. Adjust the "World Size" settings as you want them (no. of tiles, no. of civs per map-size, etc.), and save the new .biq under a new name.
You should then be able to use that .biq to play similar games, but on random maps.
I haven't really explored this option in the Editor, but as I understand it, these are intended to be used to make some additional adjustment to individual nations' relationships with e.g. each other (more or less friendly), technologies (more or less likely to pick as the next research-project) and buildings (more or less likely to build earlier), etc. AFAIK, the flavour-settings don't make a huge difference, though.