I have worked a bit in the past with third party sign-up forms, particularly building forms in MailChimp to be embedded into client sites. I have found the MailChimp URL has a tendency to scare off potential signups. You might want consider getting the outputted code instead of linking to the MailChimp forms. That way you can keep the URL and template the same as the rest of the site.
It should only take you a few minutes if you have already designed the form you want. The steps are as follows:
1) When you are logged in, go to “lists”, click “design signup forms” on the left hand side and pick your form
2) Change anything you might want on that form and click “share it”
3) Under “Create form HTML” click “Create HTML code for a small subscribe form
4) Click on classic form and make sure “only required fields” is unclicked and “all fields” is clicked.
5) Create embed code and paste the code on the page.
There will be about 15 mins a form to implement the code. See: http://www.withsquirrelly.com/email/ for an example.