Check with your glass shop again, but I think glass may come in just even metric intervals.
Glasscages.com lists 3/8 glass as 10mm thick.
Otherwise the thickness should be fine (ADA 75P uses 8mm glass). No need to get anything bigger.
My silicon did call for a gap of 1mm for the silicon to cure correctly. You just lift up the planes and push them in place. Duck tape will do to hold them in place. No pressure needed at all!
I did bevel my bottom panel as well with a rather ruff finish. My rational was to give it more bond area this way. But I never read any recommendation regrading that. Check with the Australian person (wendle) from scolley's thread, he is an expert on this. His comments are very valuable in the thread for a successful build!
BTW, there are two ways for the seam, the one scolley used and the way ADA builds tanks.
1. taping off a wider seam (scolley)
2. removing any excessive silicon beyond the glass contact areas (ADA and mention from wendle) I did not feel comfortable for the bottom with this
Good luck!