The order of vehicles in a convoy is supposed to be determined by their order in the group. 1 first, 2 second and so on. This is usually, but not always, the same as the rank order. Remember that commander is always one rank higher than the gunner who is one rank higher than the driver. IIRC you select the driver's rank in the mission editor. It is different for wheeled vehicles.
Wheels and tracks often don't mix. The best solution is to play with vehicle combinations until you find one that works, and then make your mission fit. Using loons as drivers rather than creating the vehicles directly does seem to help as you have discovered.
Often however, the problem is the mission designer not fully appreciating the rules, which are complex. Experiment till you really understand how it all works - it will stand you in good stead for the future. Until you forget.