I had to give the book I was borrowing back but we're playing Saturday and I wanted to finalize some things on my PC.

I know you can't layer armor, or at least not well. But can anyone have the 8/6 armored jacket? Or do you need an agility 8 or something to wear it?

You'd need body of 4 or higher to wear without penalty.
Just another reason why trolls will be nigh unstoppable.

Is it twice body for the highest armor rating or what?

yup

Does Orthoskin, helmets, ballistic shields, etc all count?

If it doesn't a human with 5 body can wear the best armor.

..without being slowed down a bit, yes.

Helmet and Shield do count, implanted Armor not.

they count but they also count against the best you can wear. take the urban explorer jumpsuit, 6/6. the average joe can wear this no problem. Add that 0/+2 helemt and suddenly they have a penalty.
right, bonus armor from powers and implants don't count against you. (so much for simplified system)

If you have 3 body and 8 armor what penalty do you take? -1 or -2?

-1

3*2=6

8-6=2

-1 per two points or fraction thereof.