Well, if you wanted to go that route, there's nothing wrong with making a syrup from butterscotch candies and water and adding to carbonated water. Just get about a pound of those bright yellow butterscotch buttons, crush them and heat in about 1 to 1 1/4 cups water until dissolved, then you have syrup for 1 gallon of soda.
I've done that with cinnamon imperials and it works just fine.
If you're going to ferment that, you can use the full gallon of water to dissolve the candies and it will be quicker. If you're looking for a cleaner taste, you'll definitely want the yeast nutrient. I think I used Fermaid K though that link is for a full pound, which would last through well over 400 gallons of soda. 1g/gallon should be sufficient.