I’m gonna take a look to the Domino arc on Disney+ right away so I can think of better feedback. I think “Rookies” has to stay and not only because it’s my favorite season 1 episode, but mainly because: it makes for a good establishing character moment for Rex and Cody (Rex in particular being of great relevance for the Dominos’s future arcs), Grievous and Ventress are shown to be planning an attack on Kamino as depicted in the next episode, and Hevy’s death could really hit the beats as a typical end-of-act-two downer moment.

Clone Cadets could definitely be trimmed somewhat and serve as a cold open. I’d personally like if the first training scenes where they fail could be merged and have at least one moment where the drill instructor refers to them as a “bad batch”. Then you could only show 99’s pep talk to Hevy and the squad succeeding in their test.

If I recall correctly the Kamino invasion plans fail in Rookies, but then in ARC Troopers the Separatist proceed with it anyway. Maybe you could cut the moment where a Republic fleet forces Grievous to retreat earning a happy ending, and find some way to imply that, instead of giving up, the Separatists hasten their attack on Kamino.

There is one thing that could make merging the three episodes together feel a bit rushed, and it’s the fact Echo and Fives step up from sloppy cadets to exemplary graduates to 501st soldiers to ARC Troopers in the span of a single story.