You can poach just about any fillet, from the lean to the oily, the delicate to the dense. The only fillets that can be difficult to poach are the flatfishes, because their round shape makes them tricky to remove from the pan. You can get around this problem by halving them lengthwise.
Flaky-textured fish steaks like salmon and halibut are excellent poaching choices, and are often easier to get out of the pan than fillets. Steaks with meatier textures, like tuna and shark, are less suitable for poaching.
Scallops, with their fishlike texture, are perfect for poaching. You can also poach shrimp, but since the cooking time is so short (about 2 minutes), they don't absorb much flavor.
As with fillets, it's the shape of the fish, rather than its type, that determines its suitability for poaching. Flatfish, because of their round shape, are impractically difficult to remove from the pan. For large whole fish, like salmon, add the fish before you heat the liquid; otherwise the outside will overcook before the inside is done.