Yellow tangs get to something like 6-8" long, and are extremely active fish that need large tanks even as juveniles. It should, frankly, be sort of obvious why that wouldn't be appropriate for what is quite a small tank.
Adult size of fish needs to be taken into account when buying them. Never buy a fish you don't currently have a system to house, and never buy fish based on their current size rather than their real size.
For a 6gal, you're limited to very small fish that don't swim much. Trimma gobies, clown gobies, two-spot or tailspot blennies, barnacle blennies, small shrimpgobies like antenna gobies. Two tiny ones should be a fine bio-load once the tank is mature. My suggestion would be enough rockscape to support an antenna or Yasha goby and candycane pistol shrimp burrow system, then your choice of small, rock-perching blenny or goby.