In my opinion, eating KFC once a week is too much. If your "new lifestyle" is having salads or being vegetarian the rest of the week, you pretty much are undoing all the benefits of that in one meal.
Is it okay to treat yourself on occasion? Of course. If you stuck to some diet the whole week and at the end of the week you feel healthier or have lost weight, by all means, on the weekend maybe treat yourself to a meal where you don't count the calories, fat, or such.
As a diabetic, I try to stick to certain foods and eat healthier. No sodas, no chips, no french fries. Cut off the sweet desserts. But every once in a while...maybe once a week, I'll reward my efforts with a small piece of pie, or something like that. Even if I had the KFC, I'd eat the chicken, but no fries, no Pepsi. Or I'd have the soda with a health meal like a salad.
It's called "moderation" or "self-restraint". Ultimately, I'd say do what you want. But I would ask you to ask yourself... What was the goal of this new lifestyle change? Answer that to YOURSELF to see whether you should be having fast food takeout weekly.