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You might want to review a couple of the threads that have already discussed this. Here's one:

*link*

Pretty simple, and the rules support it.

Got a rules citation for that?

Otherwise, how do ambushes work?

A surprise round works for me.

Special Initiative Actions

Here are ways to change when you act during combat by altering your place in the initiative order.

Seems clear to me. One way causes a lot of problematic issues (see the OP's scenario), one way doesn't.

you assume that not having an initiative order stop one from doing things that will effect their later on initiative. there is no rule for that.

Special Initiative Actions

Here are ways to change when you act during combat by altering your place in the initiative order.

Ok, let's go through this:
You can perform most actions outside combat but readying an action is a Special Initiative action, which alters your initiative result. So if you haven't rolled initiative you can't use it. If multiple creatures want to do something first, that's what the initiative check is. I'm not going to argue semantics about this but the game works a whole lot better if played this way (IMHO).

So, the commander and the gunners prepare to shoot on command outside of combat but this isn't the ready action.

Combat kicks off when the PC decides to cast the spell. One side is aware, the other isn't, so there is a surprise round.

The sorcerer should get a Perception check (or possibly a Sense Motive check) to notice something is up, and to be able to act in the surprise round.

Perhaps the sorcerer notices the commander is staring too intently or sees the smoke from the burning wicks held by the gunners etc.

If the sorcerer succeeds, then s/he realises something is about to kick off and may change her action.

Everyone rolls initiative, but to avoid metagaming, this may need to be done secretly by the GM for the sorcerer character.

If the sorcerer is unaware of the threat, her action in the surprise round will be to cast the spell she declared. I know this slightly contradicts the 'can't act in the surprise round', but it is the easiest way to resolve it. (A swimming character who is about to be ambushed in a surprise round doesn't stop swimming and sink just because s/he didn't take the Swim action in the surprise round.)

If the sorcerer's initiative result is higher than the commander and gunners, she gets the spell off before they were expecting it. They can still bombard her in the surprise round of course. If the commander and gunners act before the sorcerer, they can take ready actions and then cannonade the sorcerer as she casts the spell, forcing concentration checks. The GM should probably roll individual initiative checks for the gunners (especially if it takes multiple gunners to man a cannon).