RTS Movement and grid questions

I have a few questions about RTS grid movement:

1. What is better (performance wise) A* pathfinding on a large static grid, or using a navmesh?

2. How can I generate / display a grid on my "game surface" for helping judge distance / move units? I tried using OpenGL and can generate a grid, but it is solid white, and is too apparent / visible. I wnat it closer to this:

3. When moving units, what's the best way to "center" units on the grid when moving to a position? Some units are very long, how should these be handled?

4. When moving units to the same destination, how do I pick a "pack leader" to determine where to move the rest of the units around that destination. In these photos, you see that the user clicked a single point for multiple units to move towards, and the units didn't "gather" and fight for that location, they stood in formation. How can this be accomplished with units of different sizes?