Yes the co-exist happily, Alot of people currently use them in tandum. I would recommend testing the deployment functions in DS7.1 as for alot of people DS6.9 still works better out for the deployment aspect.
We currently run 1 DS6.9 Server and 1 CMS 7.1 Server with about 20 CMS Site Servers (serving Task, Package and Monitor Services). On some of those site servers we are also running the PXE Services for DS6.9 so that we can build across sites. It definately works fine but for me currently DS7.1 isn't ready to replace 6.9.
I'm not sure if by Deployment servers you are talking about PXE Servers or full DS Servers, but if you are talking DS Servers it may be worth reading the below, its a guide to running 1 DS Server and multiple PXE servers: https://www-secure.symantec.com/connect/articles/leveraging-pxe-servers-transparent-image-and-software-delivery-deployment-server-6x