A guinea pig’s daily diet should consist of unlimited hay, ⅛ cup of pellets per guinea pig, unlimited water, one cup of leafy green vegetables per guinea pig, and small amounts of fruits and other vegetables. Ideally, their diet should have a variety of different fruits and vegetables. In addition to this, one guinea pig needs 25mg of vitamin C a day to prevent diseases and illnesses such as scurvy. This can be provided with a square inch of orange, or with a commercial supplement.
Here is a link to a chart on what plants guinea pigs can and cannot eat. Guinea pigs will need about one cup of leafy greens a day, along with 25mg of vitamin C.
I have allergies too! I put the hay in a plastic bin in the garage. It is sealed to prevent mites from entering, and my garage is dry enough to store hay. It is recommended to put the hay in a place that is well ventilated, and/or a place you do not visit regularly. Air filters also help with allergies, so putting one next to the guinea pig cage may help. Wear a mask when coming in contact with the hay, and remember to wash your hands afterwards.