Longtime Curio user here
Use the Welcome to Curio project to get familiar with the terminology and the features. Use the Zengobi forums for answers to specific questions. The developer responds very quickly and the users are a good bunch
Your system capacity os more of a limiting factor than Curio itself - how much memory, speed of disk etc. You can make a project as large as you like.
Yes, in that it takes time to load or reload the content from disk. There’s nothing in Curio that makes that process significantly slower than other apps. You can store media files as links rather than embedded if you decide to keep the size of the Curio project file down.
I think you’ll end up doing that - given what you want to do, a single project could well become unwieldy . That said, if you want to reference/link between old and new you could start with a single project
You can, but you need to think about how you want to work. Curio excels at the visual representation of content and links: if you don’t need or want that in some areas, you can use Obsidian (or anything else) to collect and organise text. Given that Obsidian keeps its content in individual files, you can link to any text file from Curio if you need to. Personally, I wouldn’t mix the two, because the workflows are so different, but that’s a personal choice