I searched docs for these things but can't find answers
1) How to drag shape and preserve aspect ratio. Used "shift" while dragging in other apps but can't get this to work
2) How to drag shape without snapping to guides (also sometimes shift or alt)
3) How to copy or alt-drag to copy a shape without "linking" it to the original (so that changes to the new shape don't affect both shapes)
Chat about the original Pixelmator.
The trick here is to press the Shift key after you start drawing the shape because that key is also used to combine shapes when pressed before starting to draw a shape.
Pressing and holding the Command key will do this and this works for snapping when using any tool at all.
I don't think there are any ways to Option-drag on the canvas and put the new shape on a separate layer, so your options are:
1) Option-drag, then Command-X and Command-V to cut and paste
2) Option-drag in the Layers palette
3) Shift-Command-D (duplicate)
If the shape is a component of multiple shapes on that layer, option number 1 is the only one that will work. If not, I'd probably use option 3.
Hope that helps!