New Floral Stems

The stems are flexible: bend them rather than cutting them, so the flower stays usable in a taller vase. Make the bend low, below the first leaves, where it won't show.

Yes, as long as you stay within the same color range. Heights, on the other hand, benefit from varying: it's the difference that gives the bouquet depth.

Dust them with a feather duster and keep them out of direct sunlight, which eventually fades the colors. Store them upright, never lying flat under other objects.