How much pulling force will this plug withstand in gypsum bricks?
Is it really worth engineering a coat rack as if you're building a bridge?  

Do this as an experiment.  Choose a spot that will be behind the coat rack but not where you'll put an anchor.  Install one of the anchors, screw in the screw, and see if you can get it out with a medium pull on a claw hammer.  (Put a scrap of wood between the wall and the hammer so you don't damage the wall.)    If you can pull it out without a lot of force, use a stronger anchor.  

My guess - in soft gypsum brick, you'll want something longer.