Sietch - If you are talking about a 55cm high tank, your long sides (front and back) need to be 12mm, if you height dimension is 65cm, you need something bigger - maybe 14mm. Take a look in
here . It's all there on the last page.
Having done clear silicone now, if I were doing very small seams all the way around (I would), next time I think I'd go for black silicone. I think. It doesn't show algae as well.
I don't remember where I got mine. Google it. But
www.grainger.com and
www.hmcelectronics.com both carry it. Don't know where they will ship though.
As for the bulkheads vs. not, canister vs. sump, I'd rather not get into those questions too far in this thread - a good search here will yield a lot of thought on that. But suffice to say, next time I'm going to be looking at a normal sump setup real hard. I've personally tanken in-line about as far as it can go, and sumps seem a lot easier. And drilling bulkheads massively complicated the build and test process. You'd have to read my thread for evidence of that.
Good luck with the PM.