At my utility (which has both fossil and nuclear units) they give the same test to everyone, but depending on your score, you 'qualify' for various positions. For example, you can score well enough to be a fossil plant operator, but not do well enough to be a nuke operator.
There isn't any official study guide for the EEI tests, most people recommend using an ASVAB practice book. It worked for me. One of the old hands on here suggests that you get a book with at least three practice tests in it. Take the first test at your leisure, but time it. Then take a different test but try to shave five minutes off. And do the same thing when you take the third one.