I have a yellow watchman goby with a clown goby, with a lawnmower blenny & a mandarin dragonet. Everyone's happy & fat in the 35g hex.
Also, firefish are more mid-water hoverers, while golden-headed gobies are sand-dwellers so they occupy different ecological niches in your tank. They also look completely different from each other so that will also decrease the likelihood of conflict or territorial battles. Purple firefish are a bit more expensive but I think they're much more attractive fish
In a 20g, the main worry would be the small tank's carrying capacity. Very few fish/low bio-load can be kept in that small a volume of water
, especially if you subtract the volume lost to substrate, LR & corals.
Anthony