You'd probably enjoy 14' tubes if you want to do multidays. You can do multidays or overnights on 12' tubes if you pack light.
If it's mostly for day trips, a 12' cat would be more fun.
I'm new to cat boating, too. Been kayaking 15 seasons, rafting 11, just took my first cat run yesterday.
It's a blast. Waaay more maneuverable than a raft, not quite as maneuverable as a kayak, but doesn't take nearly as much skill.
I'd go with 1 1/4" tubing for a bit more stiffness/strength for your crossbars, or double up your 1" for more strength (make a tunnel hull/truss between the tubes).
0.065" would be a very appropriate wall thickness for steel.
I built mine from 1 1/4" EMT conduit, which is about 1.5" and ~0.065" wall. Standard precautions apply about welding galvanized.
Here's some info on painting:
1-3/8" EMT Pipe For Frame Build
These are sick:
So are these.
MADCATR Creations - Custom Fabricated Stainless Steel and Aluminum Rafting Equipment
Best ideas are:
Spread out foot bars (as opposed to both feet together in the middle)
Adjustable oar towers
Folding Oar towers
You do not need the tubes to design the frame.
My preference is modular for a cargo boat. Fullsize and fully welded is just too huge for storage or transport.
Could go breakdown or welded for a play frame...your call. If you want a lot of fine details, it's probably easier to go welded instead of breakdown/modular.