that myth needs to die. i'm not sure where/who started it but many people including myself had great results w/ acrylic/plexi + glasstank. In fact, I had to remove a "mistake" piece and you would not believe how hard it was to remove. i used DAP silicone. http://rjhaudi.tripod.com/sump/
I too made a sump that was glass with plexi inserts, I have had no issues with it. In fact I had the same problem taking it apart when I mesured something wrong, by 1/16th of an inch. You just have to be patient and give the silicone enough time to fully cure. I let mine sit for nearly a week before I added water too it.
the sump doesnt have much pressure pushing on the panes anyway, it doesnt need to hold very well. plastic would be just fine, either way actually should work great, even if the acrylic doesnt stick great, if it just barely sticks, it should still be just fine.
i have 1/8 thick glass, plenty thick, just be careful, the thinner it is the easier it is to break, so if you smack in with a heavy pump, it could break, but thickness isnt really an issue since its not holding much pressure.
I ordered 1/8 from a local glass shop today. But they won't have it ready until Saturday, and I have to have water in the tank on Sunday. The stuff at Lowes is barely a 1/16" thick and I just didn't think it'd work so well.