To see how you can make units move at different speeds check this subschool mission. Basically at a waypoint it chooses (randomly) if it wants to go to the next in line, or fork off to a secondary waypoint. In mine I have a sequence of waypoints with different speeds fork-off to one specific waypoint far away. Actually, I have 2 sets, each forking of to either a north-east (iirc WP20) or south-east waypoint. (iirc WP22) The forking-probabilties are arranged such that each speed has more or less equal probability of being chosen.
http://www.gamefront.com/files/17285..._Subschool_rar