Sally Yates was fired by Trump too
That's a good point, and one I have considered myself. I see Sally Yates and James Comey as sort of a pair. If I am going to add one, I will probably add another. Comey, to some extent, set off the whole thing. He provided the pretext for hiring Mueller.
The counter argument to including them is that they were not fired as part of the Trump-Russia investigation, both firings preceded Mueller's appointment by weeks. Yates was fired for refusing to enforce Trump's immigration rules, and Comey - well for not being up to Trump's standards.
On the other-other hand: both are right at the center of the SpyGate: Comey looks like one of the main conspirators, and Yates was involved in getting the FISA warrants.
The McCabe firing, in contrast, was as a result of the OIG report, which many of us have held out hope for being the first of many people fired, indicted, convicted and jailed based on the work of the OIG.
I'm going to reconsider my previous decision not to include them based on your comment.