However, I would not recommend manual mode unless you are fully comfortable with it. Av mode or even P mode will probably be fine, and will reduce the complications, risk of getting it wrong on the day, and your stress. Go with what you are comfortable with. If everyone is relaxed, you'll all enjoy the process as well as the result. Hey, if you're really comfortable with Manual mode, for sure, use that.
Lower ISO will be better for noise reasons, but if you're in the shade, it might be worth using ISO 400 for a faster shutter speed. Use a tripod if you have one, to reduce camera shake, and also to save you carrying the camera around while you organise the people in the shot.
Probably the setting that's most likely to cause problems is white balance. Maybe I'm just picky about white balance, but auto white balance in the shade is not always great. If you can preview the location, check that the auto-white balance is doing a good job, and manually set the shade WB option if it needs it.
And yes, I fully agree with the fill-in flash!
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