Hi Peter,
In my (limited) experience, components are created from bodies and inherit different behaviors once converted. Bodies can be grouped together, whereas components can only be grouped into selection sets, for example.
Furthermore, imported STEP components exhibit further diminished flexibility. I could alter a surface or tap a hole on a regular component that I had created, whereas with an imported STEP component I cannot.
Maybe there’s some way to “unlock” the imported component that I’m not aware of? I would love to know how, if that’s possible.
When importing components to an existing design it will not affect the timeline, however it’s true that upon opening an imported component for the first time, the timeline will be disabled by default.