I thought you used to be able to use MOX fuel in other reactors (such as vanilla reactor, or realistic reactors), but that doesn't seem to be true anymore.
The only reactor I'm able to use it in is the the MOX reactor, which I'm disabling in my pack in favor of using Realistic Reactors.
Could a config be added to allow MOX fuel to be used anywhere Uranium fuel cells can be?

MOX fuel, afaik, can be used in Realistic Reactors.

Ahh yeah it seems to be. It must be another mod causing the issue.

Steps to reproduce: Factorio 0.18.22
1) Install Realistic Reactors, Plutonium Energy, and Schall Uranium Processing.
2) Place reactor from Realistic Reactors, notice you can't put MOX fuel into them.
Without Schall Uranium Processing, it works as intended, so its either that mod or a part of your mod that integrates with it.

The bug lies in RR itself. Nothing wrong with PE nor SUP.
Reasons (and solutions) explained in my reply.

I kinda lost track on this bug, my message in RR's discussion was left ignored. This problem still exists?

Yes, indeed it does.

I applied "solution 2" from the SUP-thread.

But to complicate things further, I use krastorio 2 and nuclear fuel as well.
Krastorio raises the uranium fuel cell to 50gj(!?) and nuclear fuel produces a lot of "generic plutonium" from its breeder cells and has a cheaty "mox fuel"-recipe to produces standard uranium cells.

As A) I'm not a clever man, I just edited your MOX fuel value to 100gj but also added 40 of the mentioned generic plutonium to your MOX-fuel recipe. As two of those reprocessed breeder fuel gives four generic plutonium.

If you have some notion of balancing these skewed fuel values when these mods are tossed together, I'm all ears (bc; point A)), so I can edit the recipes more reasonable.

Thanks for a great mod!