Firstly, don't go to the 90 degree angle with the gogo. There are two ways to finish the gogo off of the back, but the best is to have your hips squared to their shoulders, literally right in front of them.
Damn, That's amazing advice. Thanks, I've got a lot to learn. Also, When I'm doing a omo/gogoplata, and i hit 90 degrees, if they try to stack what will happen? I can just sweep Right?
There is no stack from the omo, their only roll is to roll out.
Attached is a basic walk through of the omoplata (so you can see the angle change) and the rollout (which, from what it looks like, was actually as intentional sweep on my part, as I started grabbing the pantleg right from the start). I finish with a triangle, actually, so I attached that pic to answer the second question.
EDIT: Since this covers basic questions, I've folded it into the Back to Basics thread.