Mechanim is fairly easy to use and creates very nice blends between different states of animation using ‘blend trees’.
Importing animation should be as simple as making an fbx file in Maya and then importing it to Unity.
The model the animation is set on in Maya does not matter, as it just looks at the bones for it.
Therefore importing a character needs to be rigged inside of Mechanim and can be done using an autorig function.
I do no know if the scale for bones and mesh matter when attaching animation to it. Do they have to be the same size?