The META keyword tag is not nearly as important for ranking as it once was. Partly do to easy spam abuse, many search engines have devalued it, preferring to "see for themselves" what your site is all about.
That being said, I've recently seen good results both with commas and with spaces (just don't use both!) But the deal is that after 8 to 10 words, the rest of the tag seems to have little impact, so there's only a few characters to make the best keyword META tag for your site's purposes.
It's not that more words are seen as spam or get you a penalty -- they just don't seem to be worth much in any direction at all.
On my sites, I can't afford to list both a capitalized and a not capitalized version of the same word, because I'm more focused on getting as many different words as I can into the tag. I think advice about using both caps and lowercase in the keyword tag is out of date, unless you have a special situation where a proper noun is your primary word by far, or maybe you're chasing traffic from just one or two highly competitive words -- and that's a tough row to hoe.
The title and description tags are much more important today than the keyword tag ... so I suggest focusing a lot of your keyword concerns there. After you generate your keyword list, revisit all your descriptions and titles, your h tags, the first words of your body copy, etc, and make sure the best kw's are worked in to these spots.
Most of all, give it your best shot, and know that the first submissions will only be the START of a process. You'll see results in a couple weeks and be able to revise and tweak your pages where the results aren't what you want. No one has all the secrets to all the engines. And even if they did, everything would change next week! So work to stay balanced about any advice you get.