We came across a situation while playing the mission Under Siege.

Rebels

Vs

Stormtroopers, E-Web, Trandosian Hunters, AT-ST, Elite Stormtroopers, Vader, and Royal Guards.

From my understanding the turn order would be

Rebel 1

Imp

Rebel 2

Imp

Rebel 3

Imp

Rebel 4

Imp

Imp

Imp

Imp

Would that be correct since the opposite is legal if there was only 1 or 0 Imps; that the Rebels would have multiple activations in a row

You were correct

Yes. However, because of this, as the rebel players, your priority is to defeat imperial groups that have not yet activated.