Since fruit takes less time to digest than other foods, is it a bad idea to add fruit to salads?

Adding fruit to salads is great as the greens will balance out the higher fruit sugar content in the fruit.
I don't think it matters which types of fruit you are mixing in a fruit salad, but of course the fruit sugar content will be higher in a fruit salad than in a green salad. It will not make you fat, but of course if your diet contains a lot of sugar, bread, cakes and other high sugar/fatty foods then it is another matter.
Aim to focus on green vegetables as much as possible. And low glycemic fruits like berries, apples, pears etc.

Mixing fruits into salad vegetables shouldn't affect the digestion rate but fruits should be eaten before you eat protein as it is digested at a slower rate than fruits.
Fruit sugar is more nutritious than white sugar but it will still behave the same way in the body and will still feed a yeast overgrowth as mentioned by healingx below.
Get a glycaemic index which will guide you to the lower sugar fruits which will aid your weight control as you will know the best to choose like blueberries and raspberries rather than mango and bananas which are higher on the glycaemic index.
Interestingly raw carrots are low on the glycaemic index, which is the rate at which your body absorbs the sugars in foods, but cooked carrots are higher.
I hope this helps.

I think fantastic I also do my diet ,Richnest of variety of fruits great for happinest , you will feel energized.
When you compare of process sugar and Fruit sugar compeletly different because Fruit sugar complex and unprocess and great for body but obviously Human body has balance whatever you eat over Human body either will send out or stock as a fat.
Murat Gecmen
Bsc( Hons) Sports science
Strenght & Functional Training expert

Perhaps we need to step back and adjust our perspective. Maybe, just maybe we are over thinking this whole health business.
Our bodies have been programmed since before we were born to process nutrients. And they will continue to do a very good job as long as we have clean air to breathe, good food to eat and untainted water to drink.
Any fruit is good fruit to eat. Don't short-sell your body's ability to do it's job.
Eat, drink and be merry. It is called the "gift" of life for a reason.