If we write {mapIm maH chaH je} for "we are different we and them".

Is this grammatically correct, or is does it violate the accord rule, which as I understand it says "the subject must agree with the prefix of the verb" ?

Actually, it's "the prefix must agree with the subject and object of the verb." You set the prefix according to the subject and object, not the other way around.

maH chaH je is probably a kind of maH, so I would expect your sentence to be correct.

-- 
SuStel
http://trimboli.name