there are a few ways you can go, different success ofc.
1. you could use maps which a made for cavalry. eg: a completly empty and large map will force the main part of the players to choose cavalry, the possibility to choose another class stays and so a few players wont have horses.
2. Admin mod (i recommand Pouls Admin Mod) ->start event. advantage: everyone is cavalry and has to stay in this class. disadvantage: no eqipment pool to choose from.
3. change troops. you can change the troops a player can choose serversided. its very difficult but possible. so you can create 3 cavalry classes foor each faction where they can choose from an itempool. disadvantage: very difficult and like normal cavalry classes: they can disable their horse.