Seemed to work and didn’t break any layout or anything, plus I see this one lets you customize style the form(s) too which is great. I know many people use mailchimp, so I just might add some bonus styling for the sign up form for this one plugin. Looks like it’s popular and has good reviews.
Getting into SEO stuff is a BIG topic…but generally keep content fresh and related to the site and pages, use keywords in your headings, content, link titles and anchor text, ALT tags on your images, etc. Try not to link to other sites too much unless they are popular sites and have high positioning themselves. Being active on social networks such as Twitter, facebook, and others definitely helps.