best thing to feed fairy shrimp is green water. yeast can work, but i find green water to work much better. ostracods(seed shrimp) will eat detritus. generally speaking, you dont have to worry about feeding them. they will eat whatever is organic and decaying in the tank.
triops are carnivorous and need a mixture of vegetable matter and protein. as a first food, green water works pretty good, and within a few days, small pieces of fresh carrot.
water fleas will also eat green water and yeast.
green water is easier to use than yeast because it is harder to over feed with green water, but pretty easy to overfeed with yeast.