Yeah, what he said ;D
Moving the enemy in position at the last minute is probably better. Your units will probably spot them before they get close enough to spring the ambush.
Also, if you are using the sebnam2 pack then you can place extra bushes without needing the editor addon. You should get that anyway
(You can place bushes by using side=empty, class=Sebnam Pack - Misc when placing units.) Having extra bushes will allow you to setpos enemy units in place without the player seeing it happen. You can then have the enemy units move slightly forward, out from behind the bush, so they can see to fire. Use
unitname setunitpos "down" to make them crawl.
The best thing to do is read all the tutorials you can and make sure you get the command reference. It's all available in the editors depot.
BTW, bushes, trees, and smoke do NOT mask units from detection. A guy hiding in a bush may be a little harder to detect but not by much. And units will fire at each other through bushes and smoke.