Printing photos one per sheet wastes most of the paper. A grid puts four, six or nine on a page.
An image that looks fine on a screen can be visibly soft on paper, because print packs many more dots into the same inch. Save your grid at the highest resolution you can rather than at a size that looked acceptable on the monitor.
Remember each photo shrinks. A photo printed four to a page is a quarter of the size it would be alone, so fine detail that was already marginal disappears. Choose pictures that read well small.
Leave margins. Most home printers cannot print to the edge of the paper, and a grid designed edge to edge comes out with one row clipped.
Four is comfortable for reasonable size, nine works if the photos are simple and bold.
Yes, leave a small gap between images to cut along.
No.