I guess it's a matter of how lethal you want your game to be. If I were playing Cyberpunk or GURPS or 40k or a horror game I'd expect ambushes to be the most day-ruiningest thing ever. In Star Wars (even scummy EotE), not so much.
Rounds can be up to a minute long yeah? I'd say the ambushers have a really good chance of getting the first shot in, but not a whole round of attacks without opportunity for retaliation/retreat.
Perhaps have the ambushers roll Stealth vs. Vigilance. Those who pass automatically get an initiative slot before the ambushees. Those who fail roll initiative as normal. (No slot-trades in the first round.)