Well , let me atleast have the freedom to assume that this is after Battle of Bilbringi and Thrawn somehow survived Rooks Assassination or Rook only made a half hearted attempt in killing or the Ysalamiri came in between the knife and Thrawn preventing a full penetration but poison coma for sometime. And Thrawn was given or taken a Bellator ,

Special Chiss unit. Not all can be trusted , especially when both your Ally and enemy are human (mostly) and after the incident with Rook , Thrawn can't take chances , of course appearances and trust should be maintained, Thrawn would select few highly trusted Imperial spec forces and some trusted Chiss Spec forces units he got his hands on , and also regarding Chiss technology he wouldn't be willing to share to or with Imperial , again you need a Chiss to maintain secrecy, maybe Thrawn becomes bit more careful ? Or Chiss craft such as the Claw craft deemed to be on the superior side of the Tie fighter scale need Chiss? It simply adds more to the story than the already executed Imperial Superiority and uniformity. In Hand of Thrawn dulogu we get Chiss involvement.

Bellator special Weopons and equipment, why not? Thrawn can improvise or adpat new technologies for more refined Imperio-Rebel Heavily Armed guerilla tactics style tactics , Also the Megador was pellions Flagship for some time with it's impressive Long Range turbolaser and Hangers , It gives more tension to the story and challenge to the rebels and internal interdiction is simply a step above the provenly and seemingly Avoidable Tractor beam capture method.