My daughter has the lego friends and they are perfect for this along with other box sets for example: lego bricks and more pink 4625, lego ultimate building 6166, and many others are fine for this but the lego DUPLO line are larger and will not fit.
As with the other lego boards that my son has, I wouldn't use them to be walked on. It's pretty sturdy as it is and my son uses it on the floor, the table, on the ground outside. It's a nice product. I just wish it was bigger......
It would probably take a table saw or hand saw... A real tool. I have never actually consider this. The problem may be how the plastic would react to being cut. You cannot cut it with scissors ! If I had a spare I would try it out for you, but I don't . Great idea though.
The duplo blocks do fit on the larger plates. they don't fit as perfectly as duplo plates but they do fit, I checked it out in a legoland. I bought larger one so I don't have to buy the other plates when my child is able to use the smaller legos.