I know that we are not allowed to use polycarbonate with a thickness that exceeds 0.07 inches, but can we stack two sheets of lexan on top of itself and screw them together to get a material that is thicker, or would that be a violation of the 0.07 rule?
You are allowed to do this.
On the topic of polycarb, what is the closest metric equivilant of 0.07’? Sadly its basically impossible to find imperial sheets of polycarb from where i am
Actually, polycarbonate doesn’t come in .07" in the US, either, but when the standard was first set to .0625" some inspectors started using calipers to measure thickness, and some standard sheets of .0625 came out slightly too thick. So that inspectors would not fail parts that were within the intention of the rule, the limit was changed to .07" instead. The metric equivalent of .07" is 1.778mm and of .0625" is 1.5875mm. I can buy sheets of 1.5mm here in the US, so that is probably the closest commonly-available metric equivalent. (Remember that the .07 is not really available and that the inch-equivalent is really 1.5875mm, so 1.5mm is pretty close.)
Yeah I’m from the UK and we use 1.5mm for most parts, although have used 1mm in the past where it is desirable to have a bendier piece.
There are several plastics other than polycarbonate on the approved plastics list, including nylon and ABS, both of which can be found (especially nylon) in a wide range of stiffnesses. A supplier like www.mscdirect.com has a bunch of different plastics available, and www.robosource.net sells ABS sheets.
Now we are a VEXU team we are certainly using more than just polycarbonate!
Please keep in mind that if you stack 4 pieces, then the 4 pieces each count individually toward the 12" by 24" piece. With that said, you can stack 100 if you can make it work. 
If you are getting close, you should come to your competitions with a 12" by 24" piece of paper with all the pieces drawn out on it to make it easy for the inspectors to pass your robot.
Are you only to have one sheet of each material allowed, or the total surface area must be within a 12 by 24 for all material used
You are technically only allowed to have pieces cut from one sheet of 12"x24" dimensions
So every material has to fit one sheet?
Or for example am i allowed to use one sheet of delrin with one sheet of polycarb
Not as far as I know. I believe that you are allowed one single sheet of one material and all pieces must be cut from that one piece
That is my understanding.
As for combining different types of materials, I would post it in the official Q & A. I think it was resolved once but I forgot what the answer was. It is not clear in a current manual. As far as the “common-sense” rule goes, then I think you should be able to combine them.
The phrase “as cut from” in <R7e> implies that you can combine the Plastics. Again, ask in the official Q & A.
https://vexforum.com/t/answered-multiple-plastics/30904/1
looks like your answer is somewhat right. as long as it sums up to a total area of 12 x 24
Correct. I would still repost it to the official Q & A. Inspectors at events are not going to look back at Old q and A’s especially with the new system where you can print it off. Hopefully once the question gets asked for multiple years they would make it clearer in the manual.