OK, do you have the carrier yet? It looks like a copy of a MARCIRAS, which is a pretty bulky carrier, I personally would get something lighter, but that's preference. I would recommend getting Pantac and Flyye for repro gear, their gear is of much higher quality, for not much more.
You could get a Flyye EPC with pouches
Or their MARCIRAS if you are set on that design
Having some form of hydration is a good idea, but since it is indoors, you can probably just go back to the staging area. Your call really, water is always a good idea though.
Pouches wise, an admin is meant to hold items like pens, maps, papers, and small flatter items, mainly administrative things, if you need a place to throw your wallet, it's not a bad idea. I would recommend either a triple shingle or 2 doubles (mag pouches). A GP pouch for any random miscellaneous items you need to store. Get pistol mag pouches if you have any spare pistol mags. And possibly a hydration pouch on the back.
Sorry for the wall of text.