Your questions are intriguing. I am somewhat familiar with the human gut microflora but I hadn't thought about the possibility of modifying it through the use of bacteriophages delivering DNA. I looked up which species are the most common in the gut and found a list in the Online Textbook of Bacteriology http://textbookofbacteriology.net/normalflora.html
This chapter on human flora has a list of the most common bacteria found in and on the body and for the lower GI tract this includes Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, Bacteroides sp., Bifidobacterium bifidum, Lactobacillus sp., Clostridium sp., and Spirochaetes. Most of these bacteria are or can be human pathogens and cause disease if they get into the wrong place. Apparently they can survive in the gut without causing disease—usually--although they may be associated with chronic inflammation as occurs with irritable bowel syndrome. This is a huge and complex area of investigation and the data are far from complete.
You said you were interested in bacteriophages that could be used as vectors to deliver genetic material into the intestinal bacteria. The only phages I am familiar with are the ones that can infect E. coli such as T2, 4 and 6 and M13. The T-even phages are all lytic, however, and will destroy the E. coli cells. M13 is infectious but does not cause the bacteria to break up.
You did not say why you want to use a phage to introduce DNA into gut bacteria, but if you want the cells to survive then you need to use a non-lytic phage such as M13—at least for E. coli.
From what I remember of phages, they are quite specific for species of bacteria, so each species in the gut may have its own particular phages. You might be able to engineer one that would have more general specificity as a vector, but that would involve research more on the level of a PhD thesis than a science fair project.
If you post again with some specifics about what you plan to use the phage for, we could help you better to design a project. Just remember that working with human pathogens would require special training, permissions and access to a lab with a biosafety cabinet to work in http://www.sciencebuddies.org/science-f ... fety.shtml