Malcolm wrote: your plan to use 0.8 mm wall, 25 mm tube sounds like it will give you a fairly light frame. I used 0.8 mm on a tadpole trike and it worked well,
For reference purposes only:
My LWB 7-1/2 foot recumbent is made with .035 wall thickness tubing, 30mm dia, cr-moly.
essensially about the same size bike frame, and feels like riding one big torsion bar suspension bike, by far the most comfortable ride ever, highly recommend crmoly for comfort, yet ridgid twist wise if you know what i mean. Good luck with the build.