well i dont like it at all and here is why
In FF we had Crystal because it was a mineral used to power up the ship (if i am not mistaken) , and after the crash, it started apear everywhere. So in post-apoc world FF was set in, that was the easiest thing to use as currency. It also had value as a mineral but also as a Exchange Unit, so the higher-ups wanted that mineral and it was a easy and effective way to get more.
Now thing about Fallout, the currency there is bottle caps because there was no way to make more, so it had a standard value, but with no use of its own. People just decided that caps shall become money.
2 ways to do it, each with pros and cons.
For the FF version:
Pros:-can get more from playing random and killing things
-makes sense in this universe
-also makes sense game wise (why tf are spiders dropping copper or silver coins?! what diet do they have? A mineral might be found inside plants and animals throu environmental exposure or mutation cause by that mineral)
Cons:-as far as i can tell, all minerals in Em8er might be for thumping, so in order to use one as a currency, well that one might need something extra, maybe a use in the lore, like rocket fuel or something.
The Fallout version:
Pros: -no inflation
-great idea (but single player games...)
cons:-having a fixed number or caps per world is gonna suck if we have a few fast exploreres that might find them all
-no sense in this universe as using remnants from the Tsi-Hu world as money is kinda...meh
3rd option might be this: In line with the Lore, 24 colonies got together and one point. They surely started trading and in order to do that, they had to come up with a currency that works for eveyone. The reapers were using that currency before and after the Enshigi attack, so i dont think they needed to change it. Could be called some basic like "Galaxy Credits/Units" or something like that. The problem again here is that GU should not come from mobs, so maybe use this as the paid currency.
For the main game, i would say try and set up a special mineral in the lore, a mineral that can be found anywhere in the world and use that as money. Because called the money Common Exchange Unit is all fine and good, but Tsi-Hu and Kaiju dont have that on them when you kill them. So money will only come from missions (which are not planned yet).