I tried putting every pre-made nation in with my created faction on a large map, but it seemed like too much and too crowded.  Now, I tend to go with Large 2 allies/4 enemies.  As long as the bears aren't committing genocide on the AI players, its fine.  Last game, they killed 5 of the 6 AI players in the first 100 turns.  I assume it was the bears because of how evil bears are.

1 same faction, 3 opposite for a small map.  So far this keep's at least most of the factions safe from the bear apocalypse.