2.75’’ are most definitely useable; in fact, they have a lot of popular ratios such as 480 rpm or 600 rpm.
2.00’’ is undiscovered territory pretty much… and I myself wouldn’t use them for anything else other than odometry pods.
The arrangement of rollers makes me iffy on their “roundness” (and I still reserve doubts about the smoothness of a ride they provide. Even the gear ratios are spread far and wide, so either you’re stuck with really slow or really fast drivetrains.
All in all, if you have 2.75’’ omnis, use them. As for 2’’ omnis, you’re welcome to try, but I would recommend sticking to 4’’