I got two rescue dogs from the RSPCA and was advised to maintain their current food, Pedigree puppy complete dry food was fine with them, then they matured to Pedigree small adult, obviously it depends on the age of the dog. I do agree however if the food is unknown to give them fish or chicken (boiled) and rice or plain mixer and slowly introduce the food of your choice over 7 days. We have a newfoundland puppy and she is on Beta puppy, I have found though with all three dogs they are better on meat based foods than chicken based for some reason.