crafts with more mass require the skirt to be stronger to provide a force to oppose the air pressure inside the skirt. The skirt will also drag more on a c raft of more mass because it will sit lower than the same craft of less mass
I see where you are going with this, but I think your gut feeling is being too cautious. I did a quick Google search on tensile strength of garbage bags and found this cool science project report: http://www.selah.k12.wa.us/SOAR/SciProj ... l_data.pdf
Note that in all the situations, the PSI listed was over a thousand PSI (and in many situations multiple thousands of PSI). 1000 PSI is approximately 7000 kPa in SI units
PSI can be hard to get a gut feeling for, since it relates force to area. The max hovercraft mass allowed is 2kg, which is ~20 N in standard Earth gravity. 1 Pascal is 1 Newton / square meter. Thus, working in reverse, 20 N / 7000 kPa = 0.28 * 10-7 m^2, or about a quarter of a square millimeter. Said another way, you'd have to concentrate your entire vehicle on a spot the size of a pencil point in order to break a typical garbage bag.
But in reality, the skirt you will use is at least dozens of square centimeter in size. Thus the force on any given spot is going to be a very small fraction of whats needed to tear the plastic.