Hydra is a Gadgeteer board, so it CAN use some of the same modules as Spider, which is also a Gadgeteer board.
But as Gus says, they are different. If you need specific features or module, you should make sure that the type of socket that it uses is available on the hardware platform you choose. I think Gus’ recommendation of Spider says that it has a wider range of more flexible connection options, plus the added feature of being able to take a prototype you build on Gadgeteer with Spider and take that into a larger product using the EMX module (that is the core of Spider).