I have a pvc feeder for my coops and it works best with pellets or crumbles. If you are getting pullets or hens(or even chicks) find out from the seller what brand of food they are on and contnue with that. As with any animal, if you decide to change the food source, you must add it gradually to the existing food until they are on the new food completely. A sudden change in diet can be very disruptive to their digestive system. Letting them into your veggie garden when you are done harvesting is a great idea. They will till the soil for you, eat any bugs in there and help turn over and fertilize stuff for you. The general rule about giving chickens stuff is that if you wouldnt eat it, they shouldnt either. Definately no rotted tomatoes. Anything with mold or black spots is a potential health hazard and can kill chickens. I have never fed mash but when I feed an oatmeal mixture, I pick up the dish as soon as they are full, even if its not all gone. Spoiled food can also kill.
Keep those questions coming and we will answer them to the best of our knowledge. Make sure you pick your friends brain and see how she raises her cheeps. May I suggest a good book I am reading call The Small-Scale Poultry Flock by Harvey Ussery. He writes about everything you will ever need to know about keeping chickens and he has been doing it for over 30 years. Worth every dollar.