My experience with the blue-spotted jawfish has been that they are rather delicate, sensitive fish that can get stressed out very easily. They tend to be rather spookish and skittish in the aquarium, which makes them pretty challenging to keep long term, although they are amazing little fish with tons of personality once they have been successfully established in an aquarium.
I wouldn’t recommend this particular species for anything but very experienced marine aquarists who have a well established aquarium with a fully mature deep live sand at their disposal.
I would recommend that you do considerable research on the lovely blue-spotted jawfish before you consider adding one to your aquarium. Once established, the seahorses and spectacular firefish would make good take mates for the jawfish, which requires only the most peaceful, passive, non-aggressive take mates.
But even so, the blue-spotted jawfish is going to be stressed when you add the ponies or the firefish for a matter of weeks until it becomes accustomed to them, and the jawfish will be stressed out and vulnerable in the interim regardless of how nonoffensive the seahorses and firefish are by nature.
You might want to consider substituting a pearly jawfish instead of the blue-spotted jawfish in your lineup, since I have found that the pearly jawfish are much more hardy and adaptable.
A bed of sand and gravel 4-6 inches deep is necessary for the jawfish to be able to construct a suitable burrow or tunnel in most cases, so in a tank that is 20 inches deep, that’s going to leave you with only 14 or 15 inches of water depth, which could become problematic.
I would generally prefer an aquarium 24 inches tall to accommodate the deep live sand bed, since you will not be able to locate the sand bed in your sump if you will be keeping a jawfish.
Perhaps you would consider substituting another very colorful and gentle, but much more hardy aquarium specimens such as a Royal Gramma (Gramma loretto) instead? The magenta-and-bright yellow bicolor Royal Gramma is a glorious aquarium fish that is very peaceful, makes a great tankmate for seahorses, and really glows under Osram GroLux or ColorMax florescent lighting.
The Royal Gramma would be perfectly happy in an aquarium with a shallow bed of live sand 1-2 inches deep.
Best wishes with all your fishes!
Pete Giwojna, Ocean Rider Tech Support