Hi Sian
Welcome to the forum!
Generally, having high GL foods on a fast day will prevent your body from entering fat burning mode, which is what you need to lose weight and for some at least of the other benefits. Also, high GL foods will tend to make you hungry again sooner than a high protein breakfast would, or perhaps a porridge breakfast which some find fairly filling.
On the whole fruits are high in sugars so for preference, I would go for a high protein breakfast (or rather I don't have any breakfast or lunch on fast days, but if I were you, is what I meant to say) rather than any cereals or fruit. However, I believe blueberries are meant to be low sugar and so are strawberries and raspberries. Grapes I suspect are high. Melon I don't know. Apples are high in carbs and of course so are bananas.
However, it is early days for you so I wouldn't worry at all. The best thing about this diet is that you can experiment to see what works for you.
You may find like so many others that with time it is easier to skip breakfast and lunch and have no calories at all until dinner time. Many of us have discovered that having a small amount of food seems to trigger the hunger pangs whereas not eating seems to leave one feeling fine.
I hope the rest of your day goes well. See how you feel and if you find the fruit breakfast works for you then go for it! Or if you would prefer to experiment...try some different options and see what is best.