Hey, native floridian here;
I've only done koi ponds outside so far, but planning a new above-ground pond for my tropical predator fish (mostly arowana's). For koi, obviously no heater. Average pond temps during the spring/summer seem to be around 78-82 (perfect for most predator fish). During the winter/fall, however, a heater is an absolute. Pond temps have gone down to under 60 before. However, this is only a few month period, so not much to worry about. And if by rays you mean freshwater rays, either buy your rays in oregon and bring them here, or kiss your dreams goodbye. I would love to have freshwater rays also; however, they are illegal in florida. You can get atlantic stingrays, which they sell as "florida sting-rays" but can only live in freshwater for a year or two, and then must be switched to salt or they will die. Native animals? You may have a problem with herons. I had a problem with a blue heron terrorizing my koi, but a fake blue heron finally kept him away. However, if your keeping aro's, the pond needs to be covered anyways. Good luck!