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Hear, hear, Eric's description is way better than mine at and
What would the answer for 16 be? If all the pairs have to add up to the same square, at least any odd square minus one would work, so for example the list {1, 2, ..., 24} gives you {{1, 24}, {2, 23}, ..., {12, 13}}, all adding to 25.
Yes, you certainly should be able to highlight multiple notebook cells, copy or cut them, and then paste them into another notebook. Creating sections or groups of cells can make it easier. Perhaps a screenshot if it's not working for you?
I don't see an easy way, but one uneasy way is to evaluate a Grid, as in: Grid[{{"\[Bullet]", Style["Item one", "Text"]}, {"\[Bullet]", Style["Item two with a very very very very very very very very very long name", "Text"]}, ...
Please see CellDingbats, as in this for a purple filled circle: CellDingbats -> StyleBox["\[FilledCircle]", Alignment -> Baseline, RGBColor[1, 0, 1]]