To my knowledge, PlayerTeammate is non-functional, not usable.
Like I said, covering these extra followers isn't impossible, it's just how "clean" you wanted it to be. One quick and dirty option would be to use the condition "GetRelationshipRank" and have it > 0
This should cover all followers, since all followers should be friends with you in order to become followers. Hopefully, that much at least is consistent.
The downside of this, it takes away the "Follower" condition. That is, you could then use it to tell anyone your character is friends with to go to sleep.
So, good example. Because very easy, very not clean.
Complexity can go all the way up to a script that checks for these follower mods on GameLoad and auto adds them to the faction of your choosing. That's more advanced scripting, but I can show it to you. Very clean, not very easy, and it also means only those follower mods you specifically include would then be covered.