If I understand your question correctly:
1. No, you can currently only design in virtual pixels, as we are focused on designing Adobe XD for screen design. If you're interested in other units, please file this at adobexd.uservoice.com.
2. You can change the padding between the repeat grid objects by hovering over the gutter and then dragging the space. I've attached an animated GIF that should help you:
I hope this helps!