I don't see a problem with the inside corner. I've built plenty of tanks, just not with acrylic.
Just build it. If it busts, then you can tell the rest of us what NOT to do!
There was a book I reviewed several years ago - I think the review was in A/C. I'll dig it up for you and mail it to you if you want. It gives precise instructions on how to build a quality acrylic tank. Following the author's method you will need a router.
Pmail me if you want it. And if I can find it!
I PM'd You Irate
Check out this site...
http://www.garf.org/tank/BuildTank.asp
very cool