PCBgogo

Hi. I'm designing my very first PCB using Altium Designer. All is going well, however I'm struggling with drill sizing. I'm trying to better understand what my manufacture would use for bits.
Let's say I have a component lead with 0.7 mm +/- 0.1 mm diameter. I will use the worst case lead size of 0.8 mm (31.496 mil), and then add in a tolerance of 7 mil for the hole. Assuming my drill file is set to imperial 2:3, then the manufacturer will select a bit such that the hole size after plating is 38.496 mil (ideally).
they say, "Our fabs have full metric and imperial sets, so they can do any hole size you specify within 2.5 mils.". Min drill size is 13 mil.
Question 1: What exactly is a "full metric and imperial set" ? Obviously no one makes a drill that is exactly 31.496 mil, so what are the bit size resolutions? Is there an official table of chart published somewhere?
Question 2: Should I be specifying PTH hole sizing in metric or imperial?
Question 3: Should I be rounding my hole sizing to 'better assist' the fab house? I'm assuming odd sizing will be rounded up.
Question 4 (slightly off topic): If I create a footprint in metric and place it on an imperial PCB layout, am I'm going to have any issue with routing (alignment on grid, electrical snapping, etc)?
Thanks in advance!