Paul said something about people judging the angels. If Satan and the angels who followed him already have their sentence, and the rest are loyal to God, who's left to judge?

Assuming your are referring to the Bible. I will maintain all answers to that....

Actually, satan and his angels were cast out of the 3rd heaven (not specific to where) but not yet given their sentence. They are able to go to the 1st and 2nd heaven quite freely. (Book of Job)

This is my deduction and not an church-based doctrinal position-
I believe when God gave Adam Eden, he was meant to have the serpent and the evil ones under his command stewardship also. That's why the serpent was there. I mean why would that creature even be there and spoil the day?
The serpent got the better of Adam and Eve when the couple were supposed to be in charge. They were banished from the garden and thrown to earth- basically into raw creation where Satan is described as the 'prince of the power of the air' and 'prince of this world'. So basically satan is leasing earth and man is just a poor helpless subject that needs a saviour for the natural can never battle the preternatural (demons and angels). Only faith in God and Jesus Christ can deliver us from the Prince of this World.

In the end when all that are saved are made perfect by the blood of Christ, men in their glorified selves would be able to stand and judge the angels (also meaning fallen angels). This means that punishment is not equal as rewards in heaven are also not equal.
This goes into the attribute doctrine that God is just.

As far as we are told, there is no salvation for the fallen angels. There is no way for them to be back with God. Unlike sinful humans (who may be ex-murderers) but can be cleansed (their sins to be remembered no more) as only God can put even our sins into the pits of forgetfulness. We can stand cleaner than the angels.
The angels are ministers of God. We are more than that. We are meant to be living in paradise with Jesus Christ.

Im not sure if angels are unchanging creatures. The Bible seems to show that they are not. Humans are changeable and respond to grace. As God made everyone unique, so too will there be free-will even in the throes of being fallen. Not every angel that fell went full-throttle being evil. This one Im conjecturing.

The final judgement has not yet been passed. The saints will judge the fallen angels.
Angels were described as innumberable and a third of them fell. Must be a lot of paperwork when the judgment comes.