Plasma Charge system

I think they'er mainly dropped but it's part of the ramndom loot kind of system just like things like the Vintage Heat sinks are. As that's often an upgrade I tend to go for so I can have at least one weapon available wher I don' thav eto rely on ammo boxes. I kinow you get one free when you first research it but after that I think it jus tmakes random appearances in drop loots

Yeah it's always handy to have a couple of heat sinks in the inventory in case you want a weapon with one. But as far as I'm awaer most if not all the augmentations stuff you get from loot drops or from your srtike team sand such once you unlock it in your playthrough but it is random.

As the others have already said, the augments drop as part of random loot from enemies. They are not sold anywhere.

As for the combination: I didn't think that these mods would actually work together
Though from my limited testing using an avenger just now, it seems that the result is a single seeking explosive bolt. The charge up does not work (It is supposed to triple the number of explosive plasma bolts). The damage increase of the plasma charge also does not seem to get applied (though I may be wrong about that), but the fire rate seems slowed down.

I have a question about using any of those augs on the Talon specifically:
Does this fire just one projectile, or are there the original 5 or 7 or whatever number it fires?

Because I know that for e.g., the Beam augment, and IIRC also the grenade and electric conduit augment a certain weapon type dependent function is used to calculate the resulting damage per projectile. That function takes the number of projectiles fired at once into account, but only for shotguns. Since the Talon is a pistol, those augments severely weaken that gun - since there is only a single beam fired.
I'm wondering if this is also true here.