Here's something people haven't yet mentioned in this thread. I love to design worm gearboxes based on big, 1/4" thick box extrusion and then pocket the heck out of it to save weight. They're really, really strong, and although they can be hard to assemble, they're really versatile units.
For example, this year we used about this much stock for creating worm gearboxes:
- 5 ft 4" x 2" x 1/4" tubing
- 3 ft 3" x 2" x 1/4" tubing
On the other hand, I doubt a rookie team would use tubing like this unless they had a CNC mill and were pretty confident in their ability to design gearboxes.
My general advice for a rookie team would be to just stock up on 1x1x1/16 and 2x1x1/16 6061 tubing. We probably used 40 ft of the 2x1 this year and 20 ft of the 1x1. You can build almost any frame you want with those two extrusions, 1/8" alu plate for gussets, and 3/16" rivets.