Snap and Align questions

Hi. I am working with a grid, and I need to align specific layers to the grid:

Snaping When I move a layer (with sub-layers), how do I choose which one of the layers is the reference to get snapped to my guideline? For example, let's say you have a layer with a circle, square, and triangle on different sublayers; which one will be snapping to the guides? 

 I want to align a shape's center with another's center. But I do not wish both of them to average in a center. So instead, I want shape A to be aligned to the center of shape B, and B is static. Is this possible? 

Thank you