Each follower has a specific list of equipment he or she is capable of equipping. You can see this list by going to the items page on the official site and clicking the follower's icon at the top; here are quick links:
Regarding the special follower items, those are equipped in their own special slot and do not replace any other item. That slot is not immediately available, it is unlocked later in the game.
Strength increases your Armor by 1 point per point.
Dexterity increases your dodge chance by 0.01%-0.1% per point.
Intelligence increases all resistances by .1 per point.
This has a number of interesting effects - most notably, because of the way Resistance works (it's functionally identical to Armor), stacking small amounts of Int on a Barbarian, Demon Hunter, or Monk can result in a dramatic survivability increase.
Resistance, like Armor, will reduce all damage you take, and, like armor, you gain substantially larger marginal benefit by increasing it when you don't have much. Since typically, a Monk (for example), will have a reasonably high Armor stat from gear and passives, and very low Int, adding just a little bit of Int via an oddly itemized piece of gear can provide a really large survivability boost - certainly more of one than trying to add Armor via Strength.
However, because everyone has a decent amount of Armor inherent in their items, increasing Strength for a similar benefit is generally not worthwhile unless you're playing a Barbarian that wants the damage boost as well, or a Wizard or Witch Doctor with a very high Resistance stat already.
Best Answer
The primary attack stats of the followers can easily be tested by swapping in specific stat boosting equipment, but you don't need to do that as the game clearly tells you when you mouse over their stats in their equipment menu that they are as follows: