Having used 1/4" Rivets on a chassis before, I have to confirm that they are probably overkill for your application. However, a proper physical analysis of the expected loads on your frame is the only way that you can fully confirm what you should be using, where, and how much.
If you want a point to start from, I would suggest a quick look at the design of the old
IFI Kitbot, as people have reported success in the past rivetting them up.