Hi,
Ans to your first ques is ;

Go to your outputsheet wherever you want the summation of all the sheets.
Type...Sum('firstsheetname:lastsheetname'!a1) and drag the formula vertically as well as horizontally.

and in future need arise to insert additional sheet just ensure that the new sheet is not the extreme right or left sheet...i.e. it should be inserted somewhere inbetween.

Hope this solves ur problem.

YM