Wikia Answers is run by a website called Wikia. Although it's confusing, WikiAnswers and Wikia Answers have nothing to do with each-other.
Yes
By clicking the signup on the website.
No you can make a wiki or account for free
Mostly. 99% of Wikia wikis allow you to use the same account. The most notable exception is Uncyclopedia. When Uncyclopedia was founded, it was hosted on its own servers. When it merged with Wikia, it already had a large number of user accounts set-up, and could've had problems with multiple users with the same name had they merged their user databases.
You have to be at least 13 years old to register for an account, or you need to get parental permission. There are no rules against reading or editing wikiHow anonymously, though, if you are under 13.
The WikiAnswers legal team are looking into the situation.
wikipedia
Wikia's population is 127.
Wikia was created in 2004.
You don't have to be an adult to have a wikia... But you have to take your wikia site seriously.
Wikia is owned and operated by Wikia, Inc. So, since they're both Wikia, I guess you could say that.
It depends on what the website host of the wiki is if two wikias have the same host then you can use the same account