OutlawJT:
It's a very interesting premise and I see a lot of potential with it. You need to get the mechanics of something like the Red Souls just right to make it work, though. There needs to be a real risk to individual players of being replaced but it also shouldn't be too likely. It seems like a tricky sort of thing to homebrew in a balanced way based on the points of reference you're using for inspiration.
No need to balance it. If a PC gets replaced, they get replaced - and keep playing as that character. The other PCs already shouldn't trust them, so they continue on that way. All the players would know about the change, so there would be rich opportunities for dramatic irony. There might come a time in the game in which it makes sense for a character to trust a PC that the player knows has been replaced (but the character can't be sure about). That would be a fun situation for all concerned, I imagine.
OutlawJT:
One last note, this does not work as a high level campaign because of true sight. Once players start getting access to that it drains all the day to day tension from the premise except for the absolute worst 'Oh Crap!' cinematic moments like when the entire war council you sit down to meet with and have peace talks or talk military strategy all reveal to be Red Souls at once and attack the party.
If the infection is aware of things like this, one of its first acts would be to disable all kinds of true sight. The thing in "Who Goes There?" did that, by killing the animals that could be used to perform tests. The thing in "The Thing" destroyed the blood supply.
One could work with the players to determine how to do this thoroughly. If they're on board with the "no easy solutions" concept, they'll want to help. It would be interesting to imagine what would have to be done to destroy, disable, subborn or discredit every easy means of detecting the infection.