You asked how old u should wait until a tank is matured well ur tank is probably never going to be matured enough, honestly. Unless you have fifty pounds of live rock, a little exageration, than you are going to have to have some other food supplies and LOTS of bottom feeders. For the mandarin goby is a very docile and slow eater and say u feed flakes, it is not going to come to the top to get it. Frozen shrimp, okay, it will sink to the goby but the other fish may just get it first, so u put more in and the goby may not eat all the food brougt down, or it may have settled and the mandarin likes to eat moving food. So now u have a bunch of crap on the floor, that's why u need bottom feeders too. Now u try live brine, well everything in my tank eats it and even my goby will come out and hunt for it so i would recommend that to u in a second but u don't want to dump in ALL live brine for the mandarin incase he doesn't get any. (I'm sorry if I'm confusing u)... Now, u want to add something for the mandarin to munch on with or without live rock. Personally, I only have a bacterium rock and my goby is fine. Now, i put it celary, romane lettuce or some other green vegetable and make it stay on the floor. This way, my fish, crabs, inverts, and goby has something to munch on in case one fish eats too much other food so the others don't get any or in case i forget to feed them cuz i'm human, i wake up late and have to go. Also, i drop in shrimp pellets for the inverts and goby. Most of the time my arrow crab will grab two pellets and hold onto them like a little child with food she doesnt' want to give up. But what's funny is that he brakes it up, let's it float into the air and the mandarin sometimes eats that. So no, i do not think u need lots of live rock, but u r going to need alot, alot, alot, alot and finally alot of patience and a very good feeding schedule to make it work.
So I do not recommend getting a goby, they also hide alot in their tanks...