Welcome to the Nano world!
I have kept 5 and 5.5g nanos in the past, I think you have made a good choice in choosing a goby.
We currently have a 12g, which houses a basslet and two gobies, as well as a PopPom crab and a few hermits.
I think you would do fine with a
yasha hasa goby or a
high fin goby in that size tank. I didn't see anyone asking, but I would recommend a cover with either of those two fish.
If you are looking for a pistol to pair with your goby, I would suggest the candy pistol rather than the tiger, the tiger will dig more IMHO than the candy, and potentially create water quality issues.
I would suggest being careful to monitor your water quality for a little while before considering a change; damsels are much less sensitive to changes in water parameters than these little guys are.