Hi Maksym,
I moved your post into the requirements section so it can have more visibility.
Steve McConnell's recommendation re: User Guide is that an clear understanding of what the user is trying to do can be a good way to get requirements. It, of course, works best in a user intensive system.
From your list, it is hard to tell without seeing the content. The functional tests should be very similar to the user guide from a requirements standpoint (but not as easy to use by the customer). Use cases can be part of SRS. The Demo video probably has similar content as the users guide and/or the use cases.
So, for a small, not critical system, I would probably have a very light SRS (high level needs), a few use cases, and some functional tests.
Anyone else take a shot?