If your coilovers are spun all the way down, with helper springs and sway bar out, you should most likely be hitting frame. You'd need a frame knotch to go lower than that. Your rears being too high can keep you up overall as well. If you haven't already, you can usually take out the rear spring perch (the ring that threads up and down) and the spring mount, which is the rubber part that is between the top of the rear spring and the frame. With this setup in the rear you only have the spring and the piece that bolts to the rear axle, which the spring sits on top of.
Other than that you aren't gonna get much lower without air. Different coilover brands go lower than others. If you have konis, you might have done all of the above and not be as low as you would be if you had raceland ultimos, let's say.
And to comment on the skidplate idea, it's not so clear cut. Some swear by skid plates, others swear at them! It does protect a lot from junk in the road, this is true. But if you hit the actual road, (a bump, or a wheel falls in a deep pothole and your car hits the ground that way) a skid plate wont do much to protect from that crushing force, and can potentially make the effect worse. I've heard of pebbles getting stuck between the skid plate and the oil pan, and with the decreased ground clearance of adding a skid plate, it was hit and drove the pebbles into the oil pan, breaking it.
I'm not saying don't get a skidplate. I'm thinking about getting one, mainly to protect my dogbone and subframe, but just remember when you are going low the most important thing is driving with a LOT of room between you and the car in front of you, and to drive slowly. Avoiding hazards in the road is your best defense
Oh, and do not cut your strut mount...
I think there are oem Polo strut mounts that are apparently shorter, and some use them to gain a little more low, but you'd have to google that to be sure.