I am making a 4 tier wedding cake with bottom tier being 14 inches which will probably weigh a total of 30-35#.  I want to use a 20 inch x 1/2 inch cake drum I bought from GSA.  Will that support this amount of weight?

Are there general weight guidelines for other sizes of these foil covered foam boards?

AI don't know of any guidelines. Personally, I double the drum to 1" for a cake that large.

I would not trust this drum for your cake as you will be having a lot of weight on it.  I would go to a store like Home Depot and have them cut me a piece of nice 1/2" plywood. If you look in that wood department you will find they have smaller pieces there so you will not have to pay for one of the large sheets. You can then cover them with the foil  or anything you want.  I have taken wrapping paper that goes with the event, wrapped the board and then put clear Contact paper over the top and it will be safe for the cake.

You could apply the price of the "cake board" to the price of the cake in case it is not returned.  I always put my business name as well as my phone number on the back along with a message Please return to (me).  I have in my contacts that the board must be returned however, I have had them thrown away. I put a separate charge on in case I don't get it back, usually 3 times what I paid for it . If it is not returned at least you have money for your trouble to get another. Good luck with your cake!