Yeah, as far as the OP is concerned, I am not even sure there would be a 100% consensus on a set of rules as to whether Pluto rules Scorpio, Neptune rules Pisces, or Uranus rules Aquarius.
First of all, I can imagine some modern astrologers saying that they don't believe in the system of traditional dignities at all, much less modern planets fitting into it. They might dislike the notion of Mercury in Pisces being a worse or less comfortable placement than Mercury in Virgo, and just suggest that Mercury expresses itself differently in each sign, but that both of those ways are valid. Perhaps considering traditional dignities to be value judgements of an ancient culture that may have no bearing today.
The thing I wonder, with regards to modern astrology is... what exactly happens to the traditional rulers? I keep picturing this funny scenario in my head... basically, when Uranus was discovered, then Uranus and Saturn had another fight over the sky, and Saturn lost this time, leaving Uranus as the god of the sky once again. Then when Neptune was discovered, he fought Jupiter for control of the seas, and managed to take his rightful position as god of the seas. And finally, when Pluto was discovered, he took over the underworld and Mars lost territory, his empire reduced to just Aries when he once ruled both Aries and Scorpio.
That leaves me with an open question... did the traditional rulers really not react to this change at all? It's just, in real life, countries don't tend to react well when others claim territory that they used to hold without dispute. Things like two rulers claiming the same territory is the kind of thing that causes wars. So, if there was such a war, how did the outer planets "win," and is there anything stopping the traditional rulers from coming back later and reclaiming their old territories from the challenger once they're in a position of strength? What happens if, say, Jupiter and Neptune are both in Pisces, Mars and Pluto are both in Scorpio, or Saturn and Uranus are both in Aquarius? Which one rules over the other? Someone has to be on top, right?
It also kind of seems like a sign having a new ruler should have more implications than it does for other planets, since the new ruler might have different attitudes towards the other planets passing through than the old one did. I don't know, maybe I'm just overthinking it. LOL.
I did think of a potential solution, though. If the traditional ruler enters the sign while the modern ruler is in it, maybe it's considered to be debilitated somehow where it would normally be domiciled, because they are now in enemy territory in what used to be their kingdom. A "foreigner in their own land," so to speak. But once the modern ruler leaves the sign, the traditional ruler can enter their old territory and "take the sign back" until the modern ruler returns... which, in the case of the outer planets, could be over 100 years. It occurs to me that the outer planets would need a lot of time to get back to their sign to actually do anything about the traditional planets reclaiming lost territory. Meaning that effectively, the modern outer planets are only treated as rulers of a sign while they are in that sign and able to repress the traditional rulers, and the moment they leave everything is back to business as usual.
It's like, picture this. Pluto only enters Scorpio every 248 years. Mars will enter Scorpio every 2 years. Meaning that once Pluto leaves Scorpio, Mars can take it back and Pluto can't physically do much about it for the next two centuries. Same with Neptune and Uranus, though to a lesser extent. If Neptune leaves Pisces, Jupiter will enter while Neptune isn't there within 12 years, and Neptune won't be back for 165 years to stop him from reasserting control. Though in the case of Uranus and Saturn, they're a lot more evenly matched. Uranus will be back in Aquarius within 84 years, and Saturn will reach Aquarius within 30 years, meaning he doesn't have much nearly as much time to rule over Aquarius without Uranus coming back to interfere, compared with the other two.