I use a ton of paper for competition models.

Be aware that the paper weights listed for products is often inaccurate. No one actually polices this information so the companies are free to list whatever they like, and who knows how they come up with a number they choose.

Often times the numbers are wildly inaccurate. Paper listed at 30 pounds may actually weigh more than another paper listed at 40 pounds. Unfortunately, the only way around this is to actually purchase the papers and weigh them yourself.

As for using card stock for fins for competition models, no, that's a really bad idea. Unless you are talking about built up fins for scale models where weight really doesn't matter.

The issue in using paper for rocket models is always a trade-off between the weight of the paper and its stiffness. The less it weighs the less stiff it will be. The only way to tell if this is a critical issue for a particular model is to actually build it and fly it. Fortunately, paper is usually cheap.