The current ability to allow specific mobs to ride other specific mobs via tags is great but suffers from some limitations in its current form. Any riders from the can_mount tag can ride any mobs from the can_be_mounted tags, preventing specific matchups to be made. It'd be great for the system to be tweaked in order to allow only skeletons to ride spiders or only illagers to ride ravagers for example. Another issue I noticed is that mobs will try to ride an already mounted mobs, resulting in unnatural mob agglomerations, mostly around spiders by default.

Other than that, congrats for your great mod !

Another issue I noticed is that mobs will try to ride an already mounted mobs

Uhm, might have missed a check here

Another issue I noticed is that mobs will try to ride an already mounted mobs

Can't reproduce that. Summoned two zombies and after one rode a spider the other one didn't try to