The reason you drop Scav Rey first is because of RT's unique that grants him 55% TM whenever an enemy gets exposed. That allows him to attack very often. Scav Rey hits harder per hit, but it doesn't matter since he goes 2 or 3x more often than her by the time your run is over.
Plus RT can constantly remove the buffs that Nihilus steals. C3PO can prevent him from ever gaining them in the first place, but Scavenger Rey can't do anything for the buffs, and her multi hit attack gives Nihilus a lot of extra speed.
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Also, if you have C-3PO, you drop RT as well, and run RJT BB8 R2 C3 Finn.