What not to feed a guinea pig

I have 2 12wk old girl guinea pigs. I want to make them a biscuit treat using cornflour, capsicum (or any other vegetable) and water. I am then planning on frying/baking it until cooked. I know that onion, garlic and potatoe are bad for guinea pigs but are there any other foods that are bad for them? Are some cooked foods bad for them? I was just talking to my friend and she said that flour was bad for them, is this true? If you seriously think that I shouldn't do this than do you have any other suggestions that I could make for them? If you have any questions or want more detail let me know... Thanks in advance!!:)

Think of it this way - in nature guinea pigs are strict herbivores and run around in the fields and jungles of South America...so they really wouldn't be eating anything that's cooked or doesn't hang low to the ground. Cooked foods really are not ideal for guinea pigs. Their digestive systems probably aren't used to it, and I imagine the nutrients they derive from raw foods are lost once cooked.

Flour, cornmeal, and oils really are not in the best interest for the guinea pig. They won't "hurt" them, but they won't get anything from them either. It's the equivalent of junk food.

If you want to give your pigs a treat, make them a salad bowl with lots of dark leafy greens and add in some fruits like oranges, melons, apples, etc. No dairy, no meat - stick to lots and lots of hay (alfalfa for your little girls) and pellets.