For the quest "Sakura, the Kitty and the Orange Marble", do I have to hunt Extra C for all the fried chickens, or can I buy some at Ant Tunnel's food vendor? On one hand, they're different items, but on the other hand, they share the same name. Would the boss cat accept different recipes?
Only the ones that you can buy from NPCs work. The easiest place to buy them is from Parn in the Floating Market. (World Tour)
I went back to check the quest to see whether Extra C's chicken is counted, but it turns out, it doesn't track the amount of chicken I have, only the orange marble I supposed to collect afterwards. Finally, to see for myself which chicken is counted, I've bought the balance to see if fat cat accepts the combination. This is the result: I have 287 vendor chicken, and 13 extra C's. In the end, I bought another 13 thai chicken to continue.
Was there no quest helper to help you count the required items? Instead of trying out extra C's fried chickens at the npc.
No, because the quest marker is for the Marble, not the Chicken. You need to give the Boss Kitty 300 Fried Chicken as payment to take a quiz. If you answer all the questions correctly, you get the Marble and if you don’t, you gotta pay Colonel Sanders (or Parn) another visit to buy more chicken and retake the quiz.