Only twice while flying with DL have I ever had to sign anything for having a soft ski bag.

As for the weight, in my experience they've usually been pretty strict about it. As a DM, i get 70lbs which is pretty hard to exceed, but I get used to just stuffing a lot of items in there (always put clothes, never have issues) and then occasionally forget that on international flights I only get 50lbs... twice I've been caught and told to pay like $100 extra and twice I've talked my way out of it.