Tryin not to sound stupid, but how do I get my units to fire simultaneously, like in the screen shots?

very easy:
just fast click on it

You have to click on your units and point on the enemy units really fast. Then they will be firing almost simultaneously.

When it comes to "slow-shooting" units like Rocket Launchers and Bombers (their "shot" takes at least 5 or even more seconds) - they should be first in the "fast-clicking queue" - then immediately go and fire with your LAV's and Tanks and when their fire will be jut in time with the arriving long-range rockets and slow bombers.

Thanks for the help

Who says MA isn't twitchy