Belegarth

That arm armor is what I want to make, perhaps not that exact look but that sort of manica style. The armor in the picture is laced together so i wanted to go that route (and because i don't have a to of rivets left and want to save them for attaching straps.)

Do people have any suggestions for what type of cord to use? Any general tips for attaching plates together?

I have seen the plates angled both up and down , anyone have any experience on which direction is better?

Poke around on-line, there are several groups that do roman re-enactment type stuff that have very detailed information on how various pieces of roman armor aere constructed along with tutorials. The biggest challenge you might face is that they were/are usually made of metal which doesn't alway translate well to leather.
http://www.romanarmy.net/manica.htm#

There's a guy in southern marches who has one arm done like that. Not sure if he's on the forums though.

Parachute cord is a good standby. I have lamellar that's been laced together with it for 2 years and no problems. Comes in a variety of colors as well.