Yeah, from P3 onwards they made the concept of wild cards a bit confusing by acting as if they were strange, unique and destined for greatness or whatever. Especially with the PQ games being like "Multiple Wildcards are in one place. That means some wacky stuff is happening like sub personas. Wooooo."
Then you have Persona 1 and 2 where everyone could use multiple personas because of course they could. You can't even use the whole "The second Persona 2 is in a new world so only certain people can be wildcards" excuse because the party members there could use multiple personas as well.
In conclusion, Atlas made wildcards weird.
Also, in terms of negotiation, in Persona 2 the party shared the persona catalogue. Anyone could use any persona, including ones others came with initially (In terms of P5, it would be like Ryuji using Carmen or Ann using Zoro. Stuff like that.) I imagine it would be similar to that. Stats were tied to the person not the persona as well I'm pretty sure.