If you got that kit (AP) you'd be good to go for the AB plate, as is it's a Sandtrooper ab plate.
Shoulder straps, I'm not sure what the latest thinking is but I use white elastic. It's hard to see evidence of any actual shoulder straps beyond the joining of the chest and back armor on the Sandtroopers. Generally it's considered to be just minus or before the ribbed straps.
Welcome to the MEPD!