I've heard that some stats like intelligent, strength, and dexterity is multiplied by 2.5 in diablo 3.
What about stats like minimum damage?
I suppose, for all those followers, all I need is increase damage and increase main stats.
I believe only the primary stats get the multiplier for followers.
You can always view the effect of stats and damage multipliers by seeing how the "damage" number changes in your follower's popup window as you equip and unequip gear from them.
In general though, most people feel that after normal difficulty, it is not effective to equip your followers for damage, as yous will quickly outpace theirs. You may get more bang for your buck by giving them +vitality and defensive stats, so they survive longer to help you with their special abilities, and giving them magic find and gold find and +exp, which carry over to your character.