Hoff: You are describing a "drop center" rim. That extra drop in the center of the rim allows the tire bead to pull over the lip at the out side. There is a steel band or wire in that bead so it will NOT stretch at all. There is a "flat" area that the bead sets on and is where a "tubeless" tire seals. A tube will fit in there just fine. If your rims are in good shape, smooth and not pitted on this "flat" area you can run tubeless tires no problem. If that area is all rusted and pitted you will need to run tubes or find another rim. Also look at the area where the valve goes. It must be smooth to seal the Valve. If it is rusted and pitted you will have to clean it up and run a tube. Drop center rims are standard and have been around a long time Tubeless tires are a bit newer but fit the same. One more thing to look for is the rivets. Old rims are not welded so they might leak at those rivets. You should check to see if they are still good and tight.Don't want that rim to pull apart. then if they are tight and leak you can seal them or run a tube. Either way will work OK. To get a good answer to the "can I use different size wheels" question. The short answer is yes. You have to keep the same size on the rear and the same size on the front. Hot Rods have run different wheel sizes for ever. Hope this helps. If I got something wrong some one will let us both know...:)

Steve H