Are all the children of a suckling woman considered brothers to the strange child whom their mother had breastfed, or just to the one who had suckled with him?

All the children of the woman who had suckled a strange child are considered his/her brothers and sisters.

Is Iddah (waiting period) incumbent on an old lady whose husband had died?

Upon the death of the husband, Iddah is incumbent on the wife regardless of her age.

I`m a cashier at a branch of the Civil Service Consumer Corporation. Sometimes, customers pay more than the due amount. Do I have the right to take that extra money knowing that I cover shortages from my own salary?

Has Islam made it incumbent on us to give our children particular names?

No, it hasn`t, but it is desirable that we give them good names.