Dorothy, I don't know anything about Enslow specifically, but I have a couple of thoughts on your question #1. First, yes, it's definitely a good idea to pitch a project that fits into an existing series. I've had assignments from other educational publishers this way. Focus on a series that is still active, with titles published in the last year or so. I've noticed that publishers usually aren't interested in continuing series that have not been active for several years.
If you have a publication history in a similar vein to what you want to write (nonfiction aimed at kids, especially if it's on a similar subject to the book you're proposing), then you could submit a query with a sample of published work. If you're really committed to this specific project idea, or if you're not published in a similar vein, then it's probably best to do the work and put together a whole packet with extras.
As for #2, I have no way of knowing for sure, but if there are guidelines up on the website, it's a good bet they're looking for new writers. If they don't, they usually say they are not accepting submissions. There would be no point for them to deal with the hassle and cost of reading submissions if they weren't actually looking.