Feeding them is pretty easy; they'll eat almost anything. The shrimp will dance on the surface to eat flake, and come to your hand for frozen food. They require the same kinds of clean, stable conditions as any other marine fish or invertebrate, but are quite hardy.
I worry that a butterfly would eat the shrimp, though.
This is a hopelessly vague question. Which butterfly? Some will, some won't. Most won't but I can well imagine one of the larger zooplankton feeders like an auriga will if it can catch a small shrimp moulting.
I think you can summarise the captive care of any aquatic creature with feed them and make sure they don't get dieseases
