It sounds like a very interesting project, though I do want to mention that many science fairs have strict regulations about what students can actually work with (keep in mind you also need to find a qualified supervisor to help you!). That said, sea slugs are invertebrates, which are not quite as controversial as vertebrates are in terms of science fair paperwork. Tell us more about the project, remember that if you do choose to do this project, especially if you would like to enter large competitions like Intel ISEF, Siemens, etc. Do be sure to look on each competition's respective website to check out the rules (i.e. like the following link)http://www.societyforscience.org/Document.Doc?id=9
Based upon what you have in mind so far, it would seem like this project would take much more time than the average high-schooler would be able to spend on a science fair project. In general, projects like this would require a substantial amount of time to discover the genetic basis of stem cell production, and then establishing successful methods of manipulating these genes in order to alter stem cell production. My personal advice would be that if you are interested in this area, try to tackle a smaller part of this problem, perhaps, say, locating a gene that is linked to stem cell production. Hope this helps, best of luck!
Best of luck!
"There is a single light of science, and to brighten it anywhere is to brighten it everywhere." -Isaac Asimov