In order to solve the hen & egg problem of a new community (no users->no content, no content-> no users), I thought about starting with the JS features step by step, meaning that

1) I would like to start with the activity stream, fed by me and "two or three" other users with latest news - kind of a blog.
2) Then add a video section
3) Then add an event section
4) Then add possibility for GROUPS
5) etc.

Is it possible to start with a very lean version of Jomsocial, and then open up features, step by step?

Yes it is.

You can turn off most features you don't want to use.
Photos, Videos, Events and Groups are all included in this list :)