I appreciate the input from you guys with more experience than I. This is still a concept and subject to changes based on information received, so please don't hesitate to tell me I'm FOS :<).
My idea for 3 persons on 14' tubes is based on Scadden's 3-person design pontoon that was superceded by his Hybrid-style U-shped tubes now very popular. Also, the Outcast 2-person boat that is on 13' x 20" tubes with more rocker than the 14' x 22" RMR's.
Inflatable fishing pontoon boats and float tubes for anglers | Outcast
I've seen both these boats on the waters I plan on floating and they seem to do well enough. They all connect (modules) thru internal sleeves in the lower (drop) rails. Also, when I say 3-passenger, 2 of those will be under 100 lbs each (my grandsons). I am also trying to design a toprail(s) that can be added (clamped) on the outside of the existing frames to give torsional rigidity at a second point. I found a good deal on 9' Cataract oars and have come up with a way to add oartowers and bigger oarlocks to handle them and stretch the oarlock centerline to 72".
Ultimately, I hope to make a frame of 1-1/4" (nom) with fittings similar to Osseous'. When the kids get bigger, the scadden frames will go back on the respective tubes as 1-man boats and they can follow me on the RMR. I realize Dave overhypes the whitewater capacities of his boats and has alienated a lot of serious WW folk, but as a simple fishing cat on lighter waters, they are a nice unit with good features. He has given me great service on the few issues that came up and I have no desire to get into any kind of pissing contest regarding his product.