can we use box2d or any other physics engine instead of physx in unity?

You should be able to use any Managed c# Engine (with the proper dependance), as long as you write wrapper around it (make your gameObject move according to the data given to you by the engine). Just drag'n drop the Managed .NET C# DLL into Unity, and use the "using LibName;" statement in script where you want to use it.

You should also be able to write c++ wrapper with Unity Pro. But that will take more time (long and painful task) and plus it will make your game not portable (difference in plugin per plateform + no plugin support in the web browser).

Yes. Some people already worked on it: http://forum.unity3d.com/threads/131141-Open-Source-Farseer-Physics-Engine-%28Box2D%29-port-for-Unity-%28WIP%29