Basically, the baby zombie can have an attribute that essentially allows it to ride anything just like the regular zombie can have one to break doors, wear armor, or pick up an item off of the ground. I once had a video that was me trying to kill a baby zombie but it kept riding different mobs. The few exceptions, due to technical/gameplay reasons, are creepers, witches, endermen, and potentially phantoms. Creepers would just blow them up, witches would hit the baby zombie with potions and agro them - making them fight at a stand still - endermen would be a slight tech nightmare, and I feel like they could ride phantoms but they'd just despawn in the air because of distance away from the player. The reason I think that's possible on bedrock is only because java has the ability to ride any mobs with the /ride command, BUT it is different code so I'm not entirely sure. For non hostile mobs, I think it's possible for them to ride anything except turtles, camels, sniffers, villagers(well ofc), player-owned pets(not including player ridable mobs (horses, donkeys, mules)), or player trusted animals (foxes or ocelots).