I just welded mine about a month ago. Stands are fine, just make sure the rear are on the tube and far out near the tires, and I put my front stands under the spring cups. I also checked the front door gaps to ensure the car was not flexed.
I tested my welds by jacking up corners of the car, introducing flex into the body by using the weight of the car, and no issues at all. The subframes did as they should, and stiffened the car like a plank.
Don't worry about the carpet, especially if your subframes are strictly the box steel type. Welding is very localized heat, and on the bottom of your box frame, not that much heat gets transferred to the floorpan.
Please be careful on the passenger side near the fuel lines though. I went extra slow, even after pulling them out of the way a bit. My jackstands aren't very high, so it was a tough weld for me underneath the car (couldn't turn my head with the helmet on, would bash it on the underside of the car), as such, my welds aren't pretty. I was always taught that it doesn't matter how pretty structural welds are, what matters is penetration and strength, grinders are your friend!