OK, just a few pointers here:
The word is canon, not cannon. And its Ghazghkull Mag Uruk Thraka, not Mad Urk Thraka (sorry, i cant abide basic spelling errors like this because i make them so often myself

)
Fluff-wise....:
Be careful when quoting numbers, its the biggest failing i see in the
BL novels, ie the question of scale. Orks, classically, arent portrayed as the uber-plot-armoured-amazingsauce killers that Space Marines are, so when you quote relatively low numbers you may be called out...just a thought.
It is
hugely unlikely that after a failed assassination attempt on Ghaz himself, that the offending party would just be outlawed. They would all be slaughtered out of hand most likely, or at the very least the Warboss Sneaky would be killed and the rest of the warband/army forced to join another Warboss. It is also hugely unlikely that a Kommando Warboss would be foiled by mere squig hounds...
there is no way in hell that 300
SpartansKommandoes are going to be cause enough for Ghaz to chase around the galaxy like some sort of 50's comedy,
especially since you have already said that they are outlawed (did you mean exiled?), as in which case they would only be hunted/killed if they tried to rejoin Ghaz's army...
I would make a few alterations if this was my fluff:
Warboss Sneaky of the Sneaky 'Uns Kommandoes learned his trade in the Tropical Jungles on Armageddon while fighting for Ghazghkull Mag Uruk Thraka in the 3rd War for Armageddon. Having been 'Ead 'Unted early by the now deceased Warboss (insert name here), he was taken into the fold of the Kommando brotherhood and trained on how to use his brutal cunning to defeat his enemies, Sneaky was seen as the Ork equivalent of a prodigal son. While honing these skills against the Reclaimers Chapter of the Imperial Space Marines, (insert name here) met with distaster, having decimated the Reclaimers he was facing, Warboss (insert name here) unknowingly ordered his troops to fire on the personal bodyguard (and hence the person) of Ghazghkull himself! unable to explain himself in time, (insert name here) was blown to pieces by the enraged Warlord, leaving Sneaky and his Kommandoes leaderless....
Sneaky took this chance to become the new Warboss, beating the other 'applicants' into submission he renamed them the Sneaky 'Uns, binding them together with a secret pact: They would leave the Waaaagh of Ghazghkull behind, biding their time, growing in strength and numbers until they can return one day and take revenge for (insert name here) on the Warlord from the midst of his own army....sworn
ot this pact the Sneaky 'Uns have waged a secret, brutal war against the Waaaagh of Ghazghkull, bleeding his army slowly but surely during its ignimonious retreat from Armageddon, all the while waiting for that one chance, that one oppurtunity where Sneaky can get behind Ghazghkull and tear his throat out.....
Of course, the self-proclamied Prophet of the Waaagh! has heard of the pact made by this band of Kommandoes, but has laughed off the the threat. What has he to fear from such a small force in the midst of his grand Waaagh...theres nothing to fear in the darkness is there?
anyway, thats how i would do it, but its up to you....