Consequences of using smaller replacement pulley/jockey wheels in the rear derailleur I have a new rear derailleur (Microshift Advent X) which uses somewhat oversized cog wheels with 12 and 14 teeth. I am known to wear these wheels during the winter season, so I am already planning for spare parts.
I have a small stash of smaller 11 teeth pulley/jockey wheels and I wonder if I could install them when the time comes, instead of looking for matching larger spares. The benefits are that the 11-teeth parts are quite more widespread, but what are the possible problems that this could create?
Here are some negative things that I can imagine, but maybe I am missing something or underestimating the consequences.

Clearance problems between pulley/jockey wheels, derailleur cage and chain passing through them. I can imagine that squeezing larger cogs could cause that, but with smaller ones the clearance should increase.

Increased chain friction because of sharper angles when links are wrapping smaller cogs. I could not care less about it. The bulk of my friction problems in that area come from clogged bushings/bearings inside the pulley wheels; something that better seals, not more teeth, should help with.

Change of the optimal chain length. I can imagine that with fewer teeth I'd need to remove a few links from the chain? Or is it the other way around, and the chain must grow?

Worse chain retention (chain jumping inside the derailleur). I wonder if this will be noticeable given how constrained the chain inside the cage is, and with clutched derailleur keeping it tensioned.
I am now in position to answer my question, as I've obtained practical results of an experiment described in the question.
I managed to destroy an upper pulley wheel when it met a stub. The rest of the derailleur was undamaged.

The damaged guide pulley had 12 teeth (not 13 teeth as I originally miscounted). I only had 11-teeth replacements from SRAM in my reserves. So I went ahead and replaced it. I also shortened the chain by 2 half-links as I felt it was just a bit too long as well.
The main adjustment that I had to do to make it work was to ensure that the cage would not touch the cassette when climbing from the next-to-biggest cog to the largest cog. The B-tension screw of the derailleur was turned until there was a gap between these parts. A smaller diameter of the replacement part brought the cage closer to the cassette, so that had to be accounted for:

During the next two weeks, the repaired derailleur worked without noticeable differences compared to how it shifted before.
I have just received and installed a replacement 12-teeth upper pulley wheel from the manufacturer (the part is not available aftermarket yet). Again, I have not noticed any difference in shifting.