There is such a vast selection of manufactured diets on the market; it is not possible to make hard and fast rules, though many feed mastiffs on the well tried tripe and biscuit method. If you do feed complete food additional supplements should not be required as the manufacturers have already added a selection - 'read the bag'.
Mastiffs in general do not require a high protein diet and many of these feeds are very high. If the food vanishes from the dish and they look good on it, you must be on the right road. Obviously young puppies' pregnant bitches and the oldies may require something a bit different. I have found it helps to wean puppies as soon as their eyes are open. They are introduced to finely minced beef or lamb, about the size of a small walnut, which is fed to them individually by hand, once a day for 2 days and then twice a day, moving them on to a manufactured puppy porridge at about 3 weeks, or when they are standing firmly. At 5 weeks they go on to tripe and biscuits. I do not give milk after they have come off mum.
For a newly purchased puppy I would strongly advise keeping the pup on the food the breeder has suggested, at least for the first few days any changes should be made gradually or you will end up with 'upset tums'. Don't be caught in the trap of speaking to everyone you know with a dog and taking a little advise from each of them, it just doesn't work. Do remember to increase the size of the puppies meals as he grows.
FRESH CLEAN WATER SHOULD ALWAYS BE AVAILABLE.
Worm regularly monthly for puppies, twice yearly for adults with additional worming if you think it necessary. Mastiffs seem to avoid most flea invasions but not all.
Mastiffs are easy to keep clean, a daily brush and ear check with an occasional bath, should keep them sweet. We clean their teeth occasionally and they have marrow bones. Cheese or liver is a good titbit, NEVER chocolate made for humans as this can be toxic to dogs.