Why don´t you simply select all units that should attack, and leftmouseclick the target?
Ctrl-Leftclick enqueues the "click command" for the selected units. As soon as the units have nothing to do, they will attack/build the target(s).
Update: Friendly targets, i.e. your own or your allies´ droids, cannot be attacked on purpose currently as far as I know. Friendly fire, usually happening unintentional, is the only way.
It might be helpful to concentrate on oil locations with trucks in the starting phase, letting the allies do their "let my tanks attack" game. Then, after sending out zillions of trucks to occupy as much oil derricks as possible, secure them with towers. Then, recycle most trucks (if you still have them then), and produce tanks to clean up the rest. You´ll have a better chance to win since you probably have more energy available then all allies and enemies.