Before you load your fixer with all kinds of extra compounds, go read what "ionic strength" means and what that means to fixers. Long story short you can really slow down fixer action if you add too much extra stuff. That's why most fixers don't include much buffering and why the weak buffering that is there comes from compounds which are needed anyway: Sulfite/Bisulfite.

If you are concerned about pH stability, think about a wash step before fixing. Citric Acid stop bathes are usually pretty dilute so they shouldn't mess up too much. I'd be more concerned about Ammonia smell from a pH 7 rapid fixer ...