Happened to find this forum looking for an answer to something of mine and saw your questions.
The armour weight leather from Tandy is usually thick enough to protect without splints/plates. If you're looking to make a Coat of Plates, you may wish to use a little thinner. I tend to use 8-10oz, so it'll actually be more flexible to fit as a garment. As for patterns and such, you may wish to peruse www.armourarchive.org which is a forum like this, but for armour. While mostly metal, there are several leather armour makers there.
Good luck,
Daithi
