Whether Collider need be combined?

I have a house model, which are grouped by some models in the 3dsMax.(door, window, wall, roof and so on).

In order to combine the models with same material as possible, I combine all models together because it use only one material.

I make some box models as house's colliders in 3dsMax(I don't choose to add primary collide to the house(gameobject) in the unity3d because it has been combined into one, not many childs of gameobjects.) After importing the house model and box models, I make the box models as mesh colliders and save the prefab.

Now there are one render house model, five box mesh colliders in the prefab.

My Question:

1. Is it reasonable to take abvtange of simple models in 3dsMax to make mesh collider in the unity3d?

2. To reduce the number of the colliders, Could I combine the mesh colliders into one collider? Which is more optimized?