From the mechanics thread:
Increased Item Rarity
Increased Item Rarity % modifiers increase the chances of an item being magic, rare, or unique. For example with a total of +100% increased item rarity, you'd get twice as many magic items, twice as many rares and twice as many uniques from normal enemies.
This modifier has no effect on the number or type of currency items, scrolls, or gems that drop.
When in a party, only the modifier from the player who lands the killing blow on an enemy is counted.
If one of your minions gets a kill, the minion's IIR is added to yours and the total is used.
Magic, rare, and unique monsters have an Increased item rarity modifier for drops.
No, multiple instances of the burning ground damage from Fire Trap will NOT stack.
Only different sources of burning damage can stack. For example, an Ignite debuff from a spell's critical hit and the Fire Trap ground burn will stack.
When 2 of the same burning sources are present the highest damage source will take effect. Multiple effects can still apply, like 2 Ignites on one mob, but only the highest damage one will actually deal damage.
Once the highest damage effect ends, if another instance of that effect was applied and still has duration left, it will then do what damage it has left. This goes for Righteous Fire, Ignite, and Fire Trap's ground burn effect. Searing Bond, a recently added skill, is an exception and the beams can stack with itself.
This is why you should try to spread out Fire traps a little bit whenever possible.
Posted by Mark_GGG, PoE developer and creator of the games skills and their mechanics.
...Ignite, RF, ground fire and now Searing Bond are all separate
sources of burning damage, and all stack. Ignite, RF and ground fire
do not stack with themselves (only the highest of each takes effect at
a time), but do with each other.
Searing bond beams stack.
As of Patch 1.0.5 Searing Bond Beams no longer stack.
Best Answer
Look at the tags for the gems. Iron Will has the Spell tag. Ground Slam has the Attack tag. So Iron Will does not work with Ground Slam. Also note that if the support gem does work with the skill gem, you'll see a letter for that support gem on the icon for that skill gem in your hotbar.