There are copies of Lotus 7 chassis drawings at http://www.diyse7ens.com. Originals used mainly 1" and 3/4" round tube, with a few pieces of 1", 3/4" and 5/8" square tube. All 18ga.
The locost is over-built compared to an original (more 1" tube, and 16ga), but (probably) less triangulated than a modern Caterham.
I can see their concern, as according to most GCR's the Caterham roll bar (and chassis) is pretty insubstantial, especially in the roll bar mounting. If Caterhams are allowed at your track, the locost should be. They're just trying to cover their ass and protect yours.