I love tube fin rockets even though most don't care for them because they are draggy and don't perform as well as regular fins. Some think they're ugly but beauty is in the eye of the beholder.
That's the exact way I do it is to start out glueing two at a time. I don't think epoxy rivets are necessary but wouldn't hurt by any means. By the time it's all said and done, you'll have the tubes epoxied at three different points. One point to the BT and one on each side of each tube fin to the other.
Cool choice for a level one rocket. You should be fine with your plan. That is the same way I attached the tubes fins to the body on my Kraken upscale. The rocket has been launched with the equivelnt of a K motor with no problems. Another thing I like to do is run some thin CA glue around the edges of the tubes to stiffen.
My "Kracken-Up" is nearing completion. I finally got the last fillet on the tube fins today. Tomorrow I can start final sanding the fillets and cleaning up the booster section. I haven't touched the nose cone for a month or so. I need to clean up the areas I used bondo to fill low spots. Then 2 layers of fiberglass and the canards. Final sand and decide on a paint scheme. Since this is my Level III attempt I'll need to get TAP approval, but right now I plan to launch at AirFest this fall. Any chance you'll be there Layne?