>>If your existing shelves are 1/4" plate, I would suggest increasing to 3/8" (or even 1/2") if the span is much more than 3 feet and unsupported at the rear (but only if your trying to cram them in and your willing to
spend the money to be "extra safe").<<

Thanks for a very informative post, Rod. I'm clueless about structural properties, but the following remarks come from personal experience:

If you're getting custom glass made for a cabinet, you might want to get the edges beveled or rounded. A square edge can cause more damage if you accidently bump a piece on it, and is itself more susceptible to knicks and chips.

Also consider the weight of the glass itself - thick glass can get heavy! With 3/8" - 1/2" thickness you could end up reducing the integrity of your mounting system because of a significant increase in weight. Especially with small diameter mounting pegs held in wood.

I'll temper these remarks by noting its clear that decent custom glass is expensive! Don't glaze over the need to insure safe transport. If you're picking up from a shop. bring plenty of blankets to avoid a paneful ride home.

Shattered,
__
/im [Exile on Fog Street]