I use both and yes you can integrate them but from what I've heard there are issues with duplicate contacts being imported into highrise. I'm currently testing that theory and will keep you posted.
I've heard the issue comes from dropping someone into mail chimp manually and then when you sync them up, if they are already in highrise it duplicates them. I dont know why anyone would put a contact in mailchimp manually once you have it integrated with a CRM

but it happens I guess.