Cooldown stacking quick Q

Small question regarding prot pally cd mechanics.
I was reading around some forum (forgot which) the other day and came by a guy stating that Argent defender and divine protection stacked additively when used simultaneously (20%+20% = 40%). I alweys beleived all cooldowns stacked multiplicatively (20% of total damage, then 20% of remaining). He said that GoAK stacked multiplicatively with each of them, its just additive when its between those two specificly.
I just wanted to ask you guys if you could confirm/deny this information.

Multiplicatively (god that is an aweful word)

Say we have a 100 damage swing and a tank stacks AD and unglyphed DivP. AD will reduce its damage to 80. Divine Protection (unglyphed) will reduce it to 64 damage.

Deleted my prior post cause it wasn't entirely accurate and I can explain things better.

What effects are additive?

1. Combat table events.
   Hit, Crit, Miss, Dodge, Parry, and Block are all additive. A talent for instance which increases your chance to Crit by a flat percentage will always have that exact effect such as Nature's Majesty. The ability Shield Block adds (up to) exactly 25% chance to block. This applies to both offensive and defensive combat events.
2. Damage reduction (static).
   Shields which absorb a static amount of damage add to the total amount shielded. If a Power Word: Shield were cast on you that will absorb 20,000 damage, and you had Blood Shield for 5,000 damage these two effects combined would absorb exactly 25,000 damage.

What effects multiply by each other?

1. Haste.
   All sources of haste are evaluated independent of each other. A 5% casting increase from a Moonkin Form's aura is multiplied by say a 10% Haste you have from Haste Rating to get a net effect of 15.5% Haste. Note that Haste stacks multiplicatively however buffs to Haste Rating such as a trinket proc do not because such a buff isn't independent, it effects a value you were already accounting for (it changes that 10% to something higher before it was multiplied by everything else).
2. Damage / healing (percent).
   All percentage based damage reductions are multipliers. If you have 50% reduction from armor then a physical attack does 50% less damage. If you also blocked that attack with a shield you take 35% of the unreduced amount (50% * 70%). If you are in Defensive Stance that also took 10% more damage off reducing it to 31.5% of the original damage (50% * 70% * 90%). If you were also under the effect of Shield Wall it would drop to 18.9% of the original damage (50% * 70% * 90% * 60%). This also applies on the other end of the spectrum for throughput. Tree of Life which is a 15% healing buff stack multiplicatively with Master Shapeshifter which is a 4% healing buff to net a 19.6% effect.
3. Movement speed.
   Effects which increase movement speed multiply together (provided they are not mutually exclusive). For example, Crusader Aura will increase the speed of a +60% speed mount by less than it would for a +310% speed mount.

I am very sure that Crusader Aura is not a flat 20% movement speed, and various addons as well as the blizzard character pane UI say that my 310% flight (310% on top of the standard 100% when running) is actually 492% movement speed, not 430%.

410 x 1.2 = 492.

So the mounted speed increase is additive, like you said, but the aura speed increase is multiplicative.

And of course my browser is freaking out at me again while I'm trying to edit that post. I'm trying to fix that. I must have missed when the change was applied to movement speed increases. They made it so they don't stack like they used to a long time ago so I'm not sure exactly when they changed it from additive to multiplicative.
*edit*
Fixed the original list.

Crusader Aura has been multiplicative since my paladin has been my main character (end of TBC to present day) if I am not totally mistaken. It may have been like this before, I don't know.