The way that randomized content works in modulestore is a hack: it copies all of the library blocks into the course and then for each user, it randomly hides all but one of them. There are a number of inefficiencies and problems resulting from this approach. That’s why for the ongoing Blockstore Content Libraries work, we haven’t looked at implementing randomized content into modulestore just yet (instead we’re focusing on non-randomized content). Once we have Blockstore-based courses, then we’ll be able to randomly assign content from Blockstore content libraries in a way that’s much cleaner and more efficient.
That said, some other XBlocks actually work OK with the current modulestore libraries + randomization, but are not allowed because they haven’t been tested (and some blocks are known to have issues). If you want to test individual block types and enable them by modifying the code, you can, as described in my answers on Drag and drop problem type not available in content library. Just be aware that you might hit bugs, depending on what sort of APIs the XBlock in questions uses.