Mesh colliders with convex option enabled doesn't work

I have a scenario where I have 3 objects dropping onto a surface, the surface originally used a box collider and the 3 objects use mesh colliders. Pre-convex the objects interacted with the surface fine but they didn't interact with each other so I enabled the convex option and immediately my objects started falling through the surface.

I then changed the surface to use a mesh collider and one object collided with the surface but the others collided with the first object but fell straight through the surface despite each of the objects having exactly the same collider and rigid body properties (other than the actual mesh). The main difference beetween the meshes is that the objects that fell though have slightly larger faces, but fewer of them (cone with 6 faces vs cube)

I am very confused and very frustrated as the available primitive colliders are not suitable for all objects in my scenario.

Are the colliders box, sphere and capsule the only available primitive colliders?

Shouldn't a mesh collider with convex enabled also interact with primitive colliders?

I read somewhere that there is a limited number of faces the mesh can have. The meshes aren't particularly complicated but if there were too many faces how would the collider behave?

Are there any other properties belonging to an imported asset that might effect the collider?

Thanks