Although tempered glass has a much higher tensil strength (10x in some cases), it does not fracture, or let me state it another way. It does fracture, but when it does, it explodes and does not crack and drip a slow amount of water like your generic plate glass does. Both tempered and plate have safety factors that really are confusing with the graphs and charts online. Glass, just like a gold chain, is only as strong as the weakest link, it has weak areas.
Tempered glass, under a serious load, need only be lightly struck with a hard object in some cases and it will blow out as it is very brittle. Plate glass will actually bow and expand, even then when the strength is at it's limit, it will crack and not blow out. Hence dripping water.
There isn't a problem with having the sides of the tank resting on the bottom piece of glass, it isn't recommended, but you can make it work just fine. Just make sure you either support the bottom glass ONLY at the outside edges under the sides, or as mentioned above, toss a 1/2" piece of insulation board under it to even out the weight distribution.
Here I go again, diving in head first!