Is epsom salt safe and effective as a prep for colonoscopy. if so, what is the correct dosage to use?

No: You should not use Epsom salt as a colonoscopy prep, unless it is recommended by your gastroenterologist. The only preps which are proven safe are the ones that are prescribed. It is important for the safety and reliability of the colonoscopy that the prep be done according to the instructions. It's not fun, but you should do it right! Good luck.