Team 2502 uses mostly 80-20 to construct our robot, while the sentiment that T-slot is heavy is correct that doesn't make it a bad building material necessarily. Our robot this year ended up at about 110 pounds with an 8 foot lift
Also included on the robot were a pair of RS775's and about 15 feet of chain. Tslot can be doable at light weights, you just have to keep in mind it is very strong. I (150 lbs) have stood on a 4 or 5" piece of cantilevered tslot bolted in the end by an end fastener and 1/4-20 screw without causing any damage. I am not familiar with square tubing but I would expect it to have different results.
I would have no qualms with supporting my weight on any of the bars on that frame.