Small knives are great for cutting up fruit and other small tasks, but if you're going to learn how to really cook on one knife, it should be an 8" chef's knife. 6" is too small to be a versatile knife. After you master 8", you won't want to be going down.
Eventually, you just might become a knife nut. I'm using 9" and 10.5" gyuto (chef's) knives these days, which I bought in the Tsukiji fish market in Tokyo. They are beautiful...
link... Masamoto Tsukiji Knives | Victory or Death in the Kitchen
And yes, deba knives are for fish prep, not for general cooking. Western filet knives are thin and flexible, Japanese debas are wide and rigid. Debas can remove fish heads easily, and require a different technique.