Without knowing something about your stills and how you intend to release the project, it’s only possible to makie a few basic suggestions. If you bring your stills in as image sequences (beyond the scope of this post) you can always change the duration of each image using time remap. But that involves a bunch of planning.
That’s what this kind of production is all about, basic and fundamental planning. How large are your stills?they should be no larger than absolutely necessary for your release format or you will bog down your system. here is an infinite number of threads here on the cow for working with still images in FCP, AE, Motion, and other applications. Even for a “music video” you must have purpose and you will want a visual climax of some kind, a reason, a reward, for your viewers. Developing and producing your story is beyond the scope fo this thread, too.
You will not want to try to edit 7k stills in a single sequence. You will want to break it down into manageable clumps. Imagine you are sending segments to a bunch of creative editors who are all working independently of each other.
You want to work in a future-proof format so you can release in any format someone might request but you still must work within the abilities of your system.
bogiesan