If you are running a reef tank you might even want to consider not having any fish at all. Even one fish can add a huge amount of pressure to the reef system, especially in a small tank and an overstocked fish tank can end up killing your corals.
Many successful micro reef aquariums don't have fish, although they do often have smaller marine creatures like shrimp.
If you want fish but can't afford a larger aquarium, consider getting a fish only tank. If you set up an aquarium with fish properly it can take a lot more than a reef aquarium can.