I've successfully kept a Mandarin in my 75 gal for over a year. I have a 3 to 4 inch SB and a good amount of live rock. Can't say how many pounds since I bought the tank used and never weighed the stuff. I later added more live rock from another used tank. He's grown steadily and is quite the chubby little fellow. No fuge, unless you can call a small plastic kitchen sink utensil container with suction cups and some rubble in it a fuge. No target feedings, this guy cruises all day, dawn to dusk and goes about his business unphased by almost anything. I allow the back glass in my tank to become overgrown with coralline and other algae, even some hair algae which I've heard can be a refuge for pods. When I look into the tank after lights out using a small headlamp, I can see tons of pods and other creatures emerging from the live rock and sand.
I added this guy to the tank about 3 months after setting it up, but keep in mind it was a previously owned set up and the live rock didn't take long to get repopulated. I occasionally see him gobble up a piece of CycloPeeze that drops to the SB but that's an exception, mostly he cruises for pods. Other than a couple of episodes of him going almost completely pale soon after I added him to the tank, he's been as healthy as can be. So strictly from my experience, if a Mandarin can do well in my 75, I'd say your 90 gal should not be an issue and your girlfriend could be right about the paranoia. Just keep an eye on the little fella and rest assured, you'll be able to tell easily if he starts getting too skinny.
