Optimizing multiple objects by combining meshes

Hello,

I'm starting game where I have multiple different size cubes which you can place "minecraft like". Also you can recolorize those cubes anytime after you have placed them.

The game is gonna be multiplayer so game will contain hundreds(probably thousands?)of objects at some points. So if I'm not completely wrong the best way is to combine objects together to reduce draw calls.

1. Can I Combine objects with different materials together?

2. What about if material is same but objects are different shapes?

Also I have been thinking something like this:

Everytime when I place new object, game checks if there is already in game same kind of objects, same color and same shape/size. If there is, then recombine meshes together. If there's not, then make new empty game object as "parent" and put object under that and wait for more same kind of objects.

Then, everytime when I want to change color, I remove target object from combined mesh and move that under the right "parent" and recombine them together.

Would be great if someone have suggestions for this.

Thanks!