As has been said, please put new parts of the story into a new post. If you don't, we will be unable to see whether or not you've added a new installment to this epic saga.
Decent so far, for a first story, but there are a few things I'd like to address (again) for future reference. As your Marine names and general grammar have already been mentioned, I shall omit them for the sake of post length.
Shas'la T'au Kais
Shas'la? If Kais is important enough to be considered a 'commander' or targeted for assassination, he/she would be of at least Shas'el rank, and probably even a Shas'o. As you seem wont to copy-paste names or concepts from other posts, I will provide the proper forms for you.
When being addressed formally, your commander will be called Shas'o T'au Kais.
When being addressed by another Tau, it becomes simply Shas'o.
When being addressed by a friend or bond-brother/sister, it becomes O'Kais.
yo
kiss arse
What is this, the Imperial Guard? Space Marines are very formal in their speech. Slang like "yo" is incompatible with how they talk, swearing even more so. It's simply not their way. Though they were once normal people, able to say 'yo' and 'kiss arse' at will, any tendency to do this would have been very much drummed out of them during their genetic induction into a chapter and subsequent brutal training and lifestyle.
which will eliminate the Tau commander and most of the rest of the soldiers
With most the Tau army dead
1) Most
of the Tau army dead.
2) I know this has already been given some mention, but; What? How can one single bomb wipe out almost the whole Tau army of this planet? The Tau defence force on Taros was estimated at eight to nine thousand Tau by Von Gustavus' tacticians, plus five thousand Kroot and over eight thousand Gue'vesa (renegade human auxilla). And despite this small size, they managed to fend off an estimated five hundred thousand Guardsmen and three companies of Space Marines, which means they must be pretty damn elite. Assuming this Tau army is of similar size, how can one bomb wipe out a force that even three hundred Marines and enough Guard to blot out the sun couldn't kill?
3) Surely the Imperium would be better off sending Scout Marines to do this job? Or, even better, an Assassin? Tactical Marines tend to be loud and noisy, Assault Marines even more so, and even the most dimwitted of sentries could not possibly fail to spot a Devastator Marine. The Imperium has much better resources for this task at its disposal than combat Marines, trust me.
Aside from this, as I said, it's decent enough and could even become a good story, with enough work.