Well, I do keep my DPs with shrimps (cherries, amanos, ninjas). DPs will definitely munch on the small shrimps and may attack larger ones depending on the individual DP's temperament.
They will not hesitate on eating guppy fry but I don't think it's good diet for them since major part of their diet in the wild consist of worms, insect larvae, snails and small crustaceans.