Mailchimp doesn’t like it if you put affiliate links in a large number of your mails. But that doesn’t mean they forbid you to do so. As long as you keep it natural there is no problem. It’s perfectly fine to mail an affiliate link now and then. Unless your intent is to spam people with affiliate links, I wouldn’t worry about it.
I use mailchimp but will be switching back to aweber. Mailchimp is good, but aweber is better. If you are starting out mailchimp is probably the best option since it’s free 🙂