I added my mandarins 2 weeks after I set up my 18" cube tank and a bit later a bluestripe pipefish male (the female followed several months later). a year and a half later all these are still alive and well and spawning, but the mandarins moved to my new 100 gallon tank about 6 months ago.
I started my 18" cube with a good deal of fresh live rock and also added a bottle of tigger pods the day I filled the tank. After one week the tank was crawling with pods all over.
So it is doable to keep these fish in small tanks, but it requires that you know what you do, and do it in the right order.
The OPs dragonet has not a good start if that tank is as sterile as it looks. He really need to get a few pieces of good live rock to seed the tank. At the same time it would be a good idea to start a few tigger pod and brine shrimp cultures to have some live food at hand.