With all due respect to CthulhuTech's line developer, that's going to be changed in my campaigns. It wrecks a lot of the point of even having multiple character types.
From what I understand, it isn't that different character types cannot be in groups together, but merely that different character types cannot be in the same character together. No multiclassing.
The problem was that the way it is set up: If you are a Mecha Pilot, you are in a completely different division, and psychics are kept away from you. If you were a Mecha Pilot before you became a psychic, well...they will find out, and you will be yanked out, and put in for special training, and even if your psychic abilities really would make you a good mecha pilot(the Somatic ones..that boost your attributes), you will never be allowed near a mecha again...because, you know, being able to psychicly react faster, with greater precision is more useful on you with a gun after they completely retrain you then it will be if you are in a 'Mecha..
Taggers cannot be psychic anyway, because the Symbiont will eat them..but, if you are a Tagger, you are with the Eldritch Society..and the Eldritch Society cannot get mecha, or even powered armour, so forget there being a Mecha or powered armour pilot in the group if there is a tagger.