No. A sept is a major planet and its associated minor planets (including colonies in other systems). For example, T'au is the Tau homeworld and a single sept. The Tau don't really have a good equivalent to a space marine chapter. A cadre is the largest permanent formation, and it's roughly on the scale of an
IG company (or a 1500-2000 point game). Larger units are composed of multiple cadres organized into a chain of command for a specific battle/campaign. While they may fight together for quite a while if the situation requires it they function more like a mixed force of Ultramarines and Raven Guard fighting together for a campaign than a single unified chapter.
Well this sept has lost hope in the greater good. After barely surviving an Ork invasion they pretty much got depressed and decided that trying to grow and thrive for the Greater good, something they've never experienced just isn't worth it.
This would be an
exceptional event, if it is possible at all. Remember that even Farsight, an exceptional once-in-a-lifetime rebel against the Tau leadership, only broke away once he was confronted with evidence that the ethereals were lying to the rest of the Tau about the threat of Chaos. And he still sees himself as serving the Tau as a whole, just in the form of rejecting the current leadership for the greater good of the Tau under his command and protection. For a whole sept to say "screw it, we don't care about anything but ourselves" would be blatantly out of character.
What makes matters even worse is the Ork WAAAAAAGH attacking their homeworld, which has forced them to flee.
This would not happen. One of the few things that can force the Tau to stand and fight, regardless of the costs, is a threat to a sept capital world. Turning their backs on their homeworld would be completely inappropriate, unless the planet had been evacuated successfully. And if that's the case then the goal would be to protect the survivors, not to die as quickly as possible.
They now launch suicide attacks against all enemies they can find, Ork or not, not for personal gain but in hopes of in the end being able to escape the unsotppable Ork horde. Squads charge into combat, for they know a bolter round to the head is better than seeing their entire world consumed by Orks.
As pointed out, this doesn't make any sense. Committing suicide to avoid their duty is not how the Tau work. The only way to salvage this at all would be to have your force cut off from the rest of the Tau Empire and any hope of resupply, and have their suicidal nature be stubborn determination to do as much damage to the enemy as possible before they are finally overwhelmed rather than taking the coward's way out. But then you have the problem that your force just wouldn't last very long. It might be a fluffy idea for a "last stand" kind of scenario/campaign, but having that kind of army fighting battle after battle just isn't going to make much sense.