You might want to start at the Music Publisher’s Association (MPA) web site. They have a FAQ that deals with some of these issues. In short, you have to write a letter to the copyright owner and explain how you want to use the music and ask their permission to do so.
In the long run you would be better to buy Sonicfire pro it has a number of libraries to fit every occasion and you can customize the legnth to what ever you want. If you have a musical bent you would do well to buy Sound Forge Acid, I have it and I just did and video journey through the mountains and was able to write my own pieces I had one section that was up on a mountain overlooking a pond and I laid in several music loops with acid and then created a track of Indian chants and singing as a low background it went over very well and with acid it really didn't take long.