See my -- suggestions below:
I know that the pages should have the following meta tags: Title, Description and Keywords
1. Are there any other?
-- The meta keywords is not read by most search engines anymore. It doesn't hurt to use it, but it will not influence your search results.
2. Is there a limit for how many words in each meta tag?
-- The Title tag should be unique for each web page and only a few words long. Try to avoid "stop" words like "the", "and", etc.
3. Does it matter what order the words are in?
-- the order does not matter, however words used should be relevant to the web page.
4. Should you use the "s" to make word plural?
-- IF an important keyword for your site is plural, then use the plural version
5. Should there be a comma and space or just comma between words? (first word, second, third) or (first word,second,third)
-- For the Title and description, it does not matter. Coma is used to separate words in the keyword meta....but that meta is not important to use (see #1).
6. What about Capital Letters yes/no/don't matter?
-- doesn't matter
7. Should sub pages have different tags or the same as index page?
-- each page should have unique keywords that are relevant to the webpage's content.
The other places it is important to use your target keywords/phrases is in "h1", "h2"...tags,
bold/strong text, in-bound and internal text links, alt image tags, and your text content.
~Roland