There may be cases where rivets join two sheets of interior skin, and there is no "rib" behind shere they join. In this case, you are just joining .032" sheet to .032" sheet, and it won't appear that there is anything behind there. You can use a very short rivet (1/4") to do this join.
In other cases, you have two sheets that are joining over a rib, in which case you can see the rib inside the hole. It may be flush, or floating around back in there. In these cases, you need a slightly longer rivet to engage it. It makes sense to have a variety of lenths availalbe to you for these different cases.