"If I have already published articles that are not in publications, can they be added to publications even if they are published?"

Yes, click "add to publication." However, some pubs don't like to republish but there is no formal restriction against this practice. I do it all the time.

"How many publications are too many to write for?

Is it considered bad form to publish an article in one publication but link another article I have written that has relevant information to another article in another publication?"

Write and publish for which ever pubs you want. No need to restrict yourself. Have fun, meet writers and editors, dig in.

"What do you do when you have submitted an article to a pub, but it’s been a week, and no editor has made notes or edits, and yet when I edit it, the message tells me that it’s with an editor. Do I remove the article and re-submit it?"

The editor(s) may be really busy but for some this is their way of saying they're not interested in the story.