I was wondering,since i recently sharked my monster rancher game into having peaches and plant eggs in the shop, if i could fead my monster ten peaches in one weak and would it have the same effect as feading him 10 peaches over a period of years?

It depends on how old it is. The effect of a peach is to turn the clock back one year. If a monster is only 3 years old, and you feed 10 peaches, once you have fed the third peach, the rest are wasted.