Here's a list of what will now be the limitations to Andy's power mimicry:
1. He can copy an ability, but has to learn its functionality. Anything that requires knowledge/understanding to achieve he can't copy until he has a way to reach that knowledge/finesse and he's trained with the ability. So for instance, he can copy the ability to see and control Rukh, but not the functionality of manipulating them into a Borg.
2. It has to be the ability of a living being, so no abilities copied from weapons or armor. I'm uncertain about spirits and the like though.
3. It has to be something that the original power copier (past life) could interact with. For instance even if you have the ability to punch a stand that wouldn't mean your ball of potential boosting and power copying energy would be able to interact with it.
4. Abilities based on the personality, soul, etc of the user cannot be copied, but the ability to use your own can. So you can't copy the "nothing can touch me" Briah, Killer Queen, Messiah, the universe arcana, etc, but you can copy the trait of possessing a stand, or the ability of being able to access your Briah.
5. You cannot copy any physical/mental stats. You weren't doing this anyway for the most part, but remember that this also includes the amount of magic you can control, and the skill with which you control it. It also includes things like knowledge of an ability and mental fortitude to withstand any drawbacks.
6. You can get around the limitation of part 1 to an extent by learning through observation, but any process too complex or with a knowledge/wisdom base too high would be out. For instance, no matter what you can't copy Griffin's space/matter/causality magic with your current souls. This is because it comes from Griffin's deep understanding of relativistic, quantum, and theoretical physics and chemistry mixed with the stupidly high knowledge base of magic granted by Aladdin and Merlin.
7. You can not copy multiple physiological abilities if they contrast. You wouldn't be able to copy greater demon physiology from a Fraudrin to get 7 hearts that all have to be destroyed to kill you, and then afterwards copy conduit physiology that grows over time. One would overwrite the other because they're based on altering the way your body functions from the norm.
8. All weaknesses of the ability are copied, except for ones that the ability itself evolved to eliminate. If it's a weakness the user overcame mentally or with another ability then you will experience it in full. If it's something the user was fine with, but would be considered a weakness for you, you will get this as a weakness (like having a hundred thousand souls constantly screaming inside you instantly destroying your mind (just an example, you don't have this)).
9.If the ability takes preparation or takes a large stock of something to use, you still need those, the items stocked up or finished preparations are not copied. So that means you don't get any of the souls Psycho has stocked up.
10. You do not know of any weaknesses or costs in an ability by default, though you can certainly learn of them through others and through "observe", if they're the kind of thing observe would tell you.
Now for the Briah, I don't know Masadaverse well, and you know your characters better than I do, so you and Monarch (cause he's our closest thing to an expert) might have to alter these to better fit Masadaverse lore or your characters' personalities:
Since Andy's characters can act independently, and have slightly differing souls, they should have similar but slightly different Briah. The Briah would have to be based around their wish to be able to constantly grow stronger, because that's how you've built up your character. If Tsurubami uses it "I want to get stronger to be the strongest" would make the most sense based on how you've presented her so far. Her abilities would bypass baseline hax resistance, and her magic power would increase over time. For Jee-Han "I want to get stronger to stay alive and protect" would make the most sense based on how he typically acts in canon. His durability would increase over time, and he could apply barriers around allies with that same durability. For Greg "I want to get stronger so these masochistic/sadistic (is there really a difference when they're the same people?) workaholics will stop bothering me" would make the most sense based on how he's been presented so far. The Jee-Han and Tsurubami personalities temporarily get replaced with Greg's, allowing him some peace. Greg's body becomes non-corporeal, while the other two fight on his behalf, becoming stronger in all stats over time (though since it's all stats it's a slower rate compared to the other two's stat growths in Briah).
This is my interpretation of how your Briah would act, obviously it's not absolute.
