tetras, cherry barbs and guppies all grow fast to there full size. In about six to eight months. The tetras might still grow uptill 2 years but much slower after the firts year..
It might look like the tank is kinda empty but once the fish grow to there abult size you'll have nice tank.
I know exactly how you feel, I have a 250gal tank with 6 very small fish, but once they are big they would full the tank.