Fruit brewing is a delicate process but is definitely worth exploring.
Aroma gives the most charm in fruit beers, and it's ideal to add it once primary fermentation is done or waning. The heavenly aromatics are just flighty so it will be diminished by CO2 in vigorous fermentation. It's best to crush bigger fruits to let the sugars and flavor seep into the fermenting beer. It will begin another fermentation. During this time, rack to a carboy, and be patient since fruit sugars might ferment lazily.
To avoid contamination, you can pasteurize the fruit through heating to 145 degrees F for half an hour or make use of campden tablets for fruit sanitizing.
The amount of fruit you need to enhance beer widely vary, so you should first have a pilot test gallon. Consider the taste you want --forceful, average or subtle. This is just a trial-and-error process and depends on the fruit and beer style. You can also make a base 5-gallon batch and ferment one gallon of that on the side with a certain amount of fruit to measure the dosage.