From my limited experience the smaller goby's tend to be very timid and you may rarely see them in a 90g. A watchmen or similar sized shrimp goby would be a safer bet for visibility in a tank that size.
I currently have a 3/4" whitecap (10 weeks) and 1.5" yasha goby (4 weeks) living as a pair with a tiger pistol in a 30g QT in my fish room. Without a shrimp and the accompanying burrow, both fish lived in total fear of everything and in hiding all the time. When the shrimp was added, he paired with the whitecap within 5 minutes of introduction, but the yasha moved into an old clam shell and looked horrible. Two days later I removed the clam shell and the yasha (looking like he was knocking on death's door) scurried into the burrow. The yasha is now looking great and the three are living in harmony.
The yasha goby now comes out of the burrow regularly, but if I open the door(12' away) to the fishroom quickly or move too quickly into or in the room, he will dart into the burrow and not come out for 5-10 mins. Anything spooks him and I am typically lucky to get within 5 feet of the tank before he goes into hiding.
The whitecap will now hang out at the entrance of the burrow (where he always hangs out) until you open the lid of the tank or get within six inches of the glass before going into hiding for a minute or two. This goby spent six weeks in my 75g DT and I saw him once every two weeks by searching at night with a flashlight. He never met up with the candy cane pistol in the DT and we ended up tearing apart the rockwork to remove them, the pistol managed to hitchike into my 100g rubbermaid sump full of rock and sand....
We have a nano tank setup now that we will be transferring the two goby's and a shrimp into in a month or two. We may transfer them into the 75g in the future once they get some size to them, but only if they become significantly less timid then they are now. Since we actually want to watch these fish interact, we felt this was the best route to take for the time being.
I should add that the fish room housing the QT has no traffic at all, which may be helping or hindering things. As well, my 75g houses a 2.5" Rusty Angel, two carpenter wrasses and a yellow assessor.
Each fish is an individual, so your mileage may vary....
