I don't see why length would be the issue. Metro have successfully operated 4x 555s coupled together out on the network for a delivery, and 3x 555s coupled together for a rescue during service hours, so 2x 555s plus the RHTT wagon (MA06) should theoretically be fine.
There is also an adapter for coupling a 555 to a metrocar, used for the first couple 555 deliveries, so I don't see why they couldn't use that for between a 555 and MA06. However, if they would need a 555 on either side, I'm not sure if they have two of these adapters, so that could be a problem.