From the feedback on my website, it appears it does have to do with the Platinum afterall.
If you have 250,000+ total resources with at least 15% being platinum, you get the full 100 point bonus. You can also get 10, 25 points if you have less than the amount required for the maximum bonus of 100. I cannot clarify this but a few people have mentioned it elsewhere.
Source: http://www.gamersheroes.com/mass-effect-3-war-asset-guide
So, the way combos in ME 2 work is that they have two components, a primer, and a detonator.
Whenever a target currently affected by a primer is hit by an eligible detonator, there's a chance that explosion occurs based on the specific combination of effects in play. For example, Biotic Charge is capable of detonating all four varieties of combos. If a target is hit by Warp first, which acts as a primer, a Biotic Combo will trigger, which explode in an area effect for decent damage against health, barriers and armor. Biotic Combos are also notable in that they are the only Combo that triggers 100% reliably, regardless of the health of the target. Others may require a killing blow, or a fair amount of luck.
If instead of charging a target affected by warp, we had charged a target that had been set on fire by Incendiary Ammo, we'd have a chance of triggering a Flame Explosion, which will set victims on fire, causing organics to panic.
There's a whole variety of combinations that can produce one of four types of combos (Biotic, Flame, Electric, Cryo), so experiment!
With regard to the various Detonation upgrades, they specifically effect combos detonated by that power. As a result, they are incapable of stacking.
Best Answer
It's not possible. You can only select one bonus power.
The powers you see in the above screenshot are your squadmates' powers, from the console version of the game. (PC has it at the bottom).
For the record, that screenshot is from Mass Effect 1...