Regarding your first question, yeah, the "iPhoto Library" is its own entity, located inside the "Username > Pictures" folder.
You can drop whatever else you want inside of the "Username > Pictures" folder, and it won't mess with your iPhoto Library.
Although your iPhoto Library is actually a bunch of subfolders and stuff, it will appear to you as a single file (called a package). Unless you right-click on it and choose "Show Package Contents" (which is not recommended), you'd probably never know it was a bunch of sub-folders containing your pictures, thumbnails, and a lot of metadata.
If you copied your pictures from your PC into the "Username > Pictures" folder, you could add them to iPhoto at your leisure.
iTunes is a little different in that your iTunes Library doesn't appear as a package. It'll be located in your Music folder, but it will appear as a regular folder.
Same deal, though. You should be able to drag other folders of music into your Music folder, and it won't mess with your iTunes Library folder. You can then add your music into iTunes as you want. I'm not sure if iTune's default preference is to make a copy of your music, or just link to its original location, so check that before you start, and change it if needed.
You can drop whatever else you want inside of the "Username > Pictures" folder, and it won't mess with your iPhoto Library.
Although your iPhoto Library is actually a bunch of subfolders and stuff, it will appear to you as a single file (called a package). Unless you right-click on it and choose "Show Package Contents" (which is not recommended), you'd probably never know it was a bunch of sub-folders containing your pictures, thumbnails, and a lot of metadata.
If you copied your pictures from your PC into the "Username > Pictures" folder, you could add them to iPhoto at your leisure.
iTunes is a little different in that your iTunes Library doesn't appear as a package. It'll be located in your Music folder, but it will appear as a regular folder.
Same deal, though. You should be able to drag other folders of music into your Music folder, and it won't mess with your iTunes Library folder. You can then add your music into iTunes as you want. I'm not sure if iTune's default preference is to make a copy of your music, or just link to its original location, so check that before you start, and change it if needed.