I do advise you to have fruits for breakfast and consume them alone. If, however, you fail at satiating the hunger throughout the day and feel extremely hunger in the evening, you can treat yourself to a few slices of fruit, but do not overdo it. Sticking to that rule is quite hard, but leads to a better digestion.
Fruits are digested in the small intestine by a special set of ferments, as they go directly through the stomach. I recommend that you consume them alone without any mix-ups even though doing the opposite won't be such a big deal.
Don't eat fruits between meals so as not to mess the digestive system up due to the fruit fermentation, decaying, or toxin production that may come out of all that.
The reason is that fruits get blocked when there happens to be another food entering the stomach so they just stay there until the assimilation of the other one is done. Don't consume sour fruits on an empty stomach since it raises the risk of having stomach issues even suffering from none at all.
Common conceptions of desserts
Desserts are typically taken as a meal that comes after the main one, however, when talking about healthy nutrition - and especially about separate nutrition - there is not such a notion at all. Desserts, such as cakes, bars, etc., have to be consumed alone, without being combined with anything since they themselves are a bad combination of sugar and starch. Sweets, containing fruit filling, chocolate, nuts and dough, are extremely hard to digest.
I am perfectly aware of how delicious they are, so enjoy them now and then without crossing the line, of course.
The most valuable desserts
Raw fruits and fresh juices are hands down the ones that will provide enough minerals, vitamins, micro and macro elements for your body. Some veggie juices may also taste like fruits. Such are cabbage, carrots, kohlrabi, etc., as you should not combine these types of juices since it won't make a very healthy mix-up juice. Breaking this rule will mean harassing the digestive system, resulting in slow assimilation of nutrients.