Dont know about interclass working (they do couple and tow each other) its unlikely things like door release/pids/etc... are compatible until the T68's are refurbished due to different control system architecture (T68's point to point 'spaghetti' wiring from cab to each component individually, M5000's use a networked approach via a high capacity central spinal data cable), but yes their theoretically capable of working in multiples of upto 4 units however the former heavy rail platforms are on average about 3 tram lengths long and the city centre stops are 2 trams lengths long. The reason the M5000's havent worked in multiple in service yet is because they dont have retractable steps so a double cant use a sloped or humpback platform (previous design was that a single unit would have full length level platform whereas a double unit required retractable steps for rearmost and frontmost doors due to half height platforms at those locations, saved pavement space.) This is being phased out however, presently all sloped platforms have been made full length except Mosley St which is due to close in a couple of months, so you should see multiples of M5000's then.
