A question is not in "Does God exist?", but rather a better questionaire is as follows: Should God exist at any point or Can God exist at any point? If you say yes to either, then why to that should or how to that can? And plus, since dealing with a God Should Exist Reason or a God Can Exist Cause, which God do you think sounds better: A) a God which will do what it should or B) a God which will do what is can. On a side note: No one teaches you "Do all you should do", but many teach you "Be all you can be". Though it is up to you to figure out any thing in wisdom.
Someone clever said that the truth doesn't have to make sense, because it has the luxury of being true. In much the same way, I suspect that whether something "should" or "should not" be true is something one would only need to apply to lies.
I think that if a good person with a good heart who respects his/her surroundings read those two statmements , both statements would mean the same thing. be all you can be and do all you should do, in my opinion should* mean the same thing.