Roaches are nasty little things. Out in the open they don't stand a chance. But as limited as their Intelligence is, a bunch of them will scatter as soon as anything big and nasty enters the room. So while you might get a couple, most will disappear through cracks. Doesn't prevent them from being a pest, and they are almost impossible to get rid of.
The kobolds won't fight in the open. They won't fight in the fort either. They'll scatter like the wind. They hide in all sorts of holes, take pot shots at things they think are weak, steal, bribe, lie, defecate in the well, sabotage the walls, (by removing or weakening supports), mine underneath the buildings, take hostages, lob stones from afar, taunt, and in general cause everyone a great misery.
I wouldn't be running 400 kobolds at a time, instead have them in units of 5-10, and have a general idea of what each "team" is doing, and their goals. If out in the open, you want to have the teams seperated by a fair distance, so the players have to decide which threat to deal with.
Also players are not too keen on using damaging area effect spells on those they are protecting, nor will they like damaging structures that are supposed to be for their protection.
As/Once the kobolds are aware of the far greater fire power of the PCs, the idea would be not to confront them, but to hastle them. Prime in this ability is to reduce the vision of the PCs through the use of smoke/fog. Concealment provides 50% miss chance, and reduces vision to 5 feet. Can't be slinging fireballs across great distances then. And it can't be penetrated by true seeing. If you don't like the idea of the kobolds starting a bonfire, have a fog be the trigger for the kobold attack. It isn't too much to expect that the kobolds should be able to predict that there will be a fog on the morn, given that they have been living at the locality for a while.
With their vision impaired, the PCs will be hard pushed to know who is attacking them and for what purpose. I propose to you that the kobolds would be doing this to harass the PCs, so much so that they feel it isn't woth the effort to stay. Slay a few of the low-life humans. Set fire to buildings. Destroy foodstocks, kill livestock. Feeding 60 people on the run is no joke.
Do not forget the attitudes of the 60 low-life humans there either. After a couple of raids, they'll be blaming the PCs for bringing them to this slaughterhouse.
Those PCs had better make good use of the hours of daylight to take stock of their situation once they enter the fort.