I generally agree with AGDTG here but will add that especially with regard to ceiling height, you should use speakers with wide dispersion. This is especially important with lower ceilings and multiple seats. Finding these specs can be almost impossible, but I believe most IC speakers of any decent quality would share this trait. So for me, as well, I’d play the long game with the larger speaker. But I know in your case just installing the smaller BS speakers to test with might be smarter. I went head first and straight to IC with RSL c34e’s.
I like your movie choices too. But will add that M&C should be watched on DVD with the DTS track and upmixed. What happened was in future releases. They neutered the LFE track with a 30hz filter. Respectfully, your current subs will appreciate that, but in the future, after you upgrade them, you’ll want to remember that.