Although I have built ply wood tanks with glass fronts, I have only assisted a glass tank builder named John Pettersen (now lives in Virginia) on a few glass tanks. This man is amazing building tanks. He built 2,000 tanks working at a manufacturer in one year once. He is an engineer and unfortunatly is too busy in that field now to work with or keep fish (Had over 140 tanks of his own in his house).
He uses 1/2 glass on the sides of a 180 and either 1/2, 5/8 or 3/4 on the bottoms. He recommends GE1200 silicone. He built my 360 8x2x3 using 3/4 and my 800 (8x4.5x3) using 3/4. He is the fellow pictured installing the glass in my 50k+ he also installed and largly built my 15k. I have never built a plexi or acrylic tank. I bought 2 acrylic tanks (500 & 600)from Aquart near Columbus Ohio (Rich Gould - owner) that I saw in advertisement in FAMA.
Good luck with your plans & hope this helps