I have read that taking gamma tocotrienols targets prostate cancer stem cells. Is this true?

No. There's lots of smoke but no fire. Every study has been done in cell culture, and nothing is proven in humans. I did a quick search and the top hits are people selling the supplements. For a scientific answer, see: http://www.Ncbi.Nlm.Nih.Gov/pubmed/11937432.
Dietary Vit E & CAP. Attempts to find benefit of tocotriences in cell cultures have been promising, though existing data on alpha-tocopherol in vitro and in vivo suggest an increase in prostate cancer risk (http://www.Ncbi.Nlm.Nih.Gov/pubmed/21990298). If you have a strong family history, discuss with your md the possible use of finasteride, the only proven chemoprevention for prostate cancer.
Unlikely. Please do not believe everything you read. Refer to reputable sources like cancer.Gov or uptodate.Com for your medical information.