Sending six screenshots means the reader has to piece them together. One tall image tells the story in the order you intended.
Screenshots at different widths stack into a ragged column with gaps down the sides. Cropping them all to the same width first is what turns a pile of pictures into something that reads as one continuous image.
Crop the repeated chrome out of everything except the first. Six copies of the same status bar and address bar running down your image is visual noise, and removing them makes the content flow.
Check the edges before sending, harder than usual. A stack of six screenshots is six chances to leak a notification, a tab title or an unrelated message.
Crop each screenshot before you join them, so browser bars and unrelated parts do not appear in the middle of the combined picture. Screenshots from the same screen are usually the same width, which is what makes a stack look neat.
Keep the gap small, around 8 to 16 pixels, and choose a background that matches the pages. Save as PNG for screenshots with text, because it keeps the letters sharp, and use JPG only when the file must be small.
Yes, the same tool stacks horizontally as well as vertically.
A tall stack can be. Compress it afterwards if you need to send it by email.
No.
PNG for screenshots, because it keeps small text crisp. JPG makes smaller files but can smudge letters.