Basic Composition Settings and Timeline Questions

I am familiar with Premiere and am learning the basics of AE and my brain is having a hard time transitioning, and I have some basic questions:

1. Are there any issues if you establish the Composition Settings before you start a project in AE vs. if you just drag graphics into the timeline animate them, and later go and change the resolution, frame rate and duration in the Composition Settings afterwards?

2. My composition/project has a duration of 30 seconds based on what I entered in the settings, and the graphic I dragged in also has this duration and I already animated it, though I am trying to shorten the duration it displays. I am having a hard time dragging the end of the file in the timeline to the left to shorten it. I also do not see a way to change the duration in Properties for the graphic so I can just manually enter time code for it. I have other layers in the Timeline and I want the entire video to have a duration of 30 seconds, though the graphic I am trying to shorten will not. How is this done?

Thanks in advance!