Hi,Welcome to the wonderful world of guinea pigs---but be warned-they are addictive!.
Piggies are grazers so food should be available at all times.Like people,some eat more than others.Piggies do not overeat as they aare selective eaters.It is normally lack of excercise rather than too much food that makes piggies obese.Keep your piggies in as big a cage/pen as you can,and if possible,give them a run out every day.Quality hay,dry grass,water,and a dry mix should be available all the time.They need at least one good serving of mixed veggies per day.Some own ers give 2 servings of veggies per day,some even more.There is a link on this forum to good aand bad veggies.There is a large variety of safe veggies and herbs.Cucumber seems to be a top favouite,and kale,cabbage(green) celery,carrots,corn and leaves,chicory,parsley,coriander ,tomato, are just a few of the veggies my piggies enjoy.Give them as much variety as you can.
I cannot coment on the dry mix you mention as I have never used it.Maybe someone else can advise you on this.I use Chudley Rabbit Royale because it does not have any chemicals.It does not have vit.c but as my pigs get loads of fresh veg,this does not matter.There are a lot of mixes to choose from.,but try and find one that your piggies like that does not have any chemicals or colourants or molasses.Also check that the protien level is not more than 12%.
Enjoy your piggies, I know you will.
Mary