What should I be feeding my dogs?

I am at a loss really. I have been having some issues lately involving the health of my dogs. My vet suggested rice & chicken to assist with diarrhoea issues that my 13 week old chihuahua is having.

As a result, I am feeding the puppy and the adult maltese/bichon a chicken, rice & vege mix. I have stopped feeding them their usual mix of Eukanuba dry food plus canned wet food.

There may be no relationship to the new diet and the recent constipation in my older pooch but thought a Flutherite may know. The older dog has been quite whiney and tight around the stomach area. Still eating and drinking but mildly uncomfortable.

Today I cooked up rice, vegetables, chicken hearts, chicken breast and raw chicken necks (all with vegetable stock).

Can anyone offer be any advice or assistance regarding diet.

PS: I would like to add that the older dog somehow managed to get into a block of chocolate the evening before last. A large quantity was consumed which almost resulted in an after hours emergency vet visit. No vomiting ensued and the only reaction was hyperactivity. Could this be related to his current issues?