[jwfoss]

A hybrid design is an excellent option, you will notice that most teams who build with sheetmetal still use "traditional" round or square tube parts. One idea that you might want to look into is the idea of nesting your drivetrain inside of a larger extruded aluminum tube. This can be seen on teams 177, 816, as well as the "Rock Box" from the 221 Robotics guys.

Other options include designing with standard C-Channel.

1/8" material thickness is common for sheetmetal and a relatively safe bet, some teams use thinner as well such as 0.090". In terms of alloys: Aluminum 5052 is great for bending and is what most of the teams that use sheetmetal use. For extrustions or flat plates/gussets 6061 is common and fine.

Again with all designs, always design within you teams capabilities... Expand your resources and then expand your design.