Currently there is not a scheduled "end of lifecycle" date for the v1 web services. We are no longer developing new functionality for it but we do make important bug fixes. My unofficial take is that we'll continue supporting v1 until there is a suitable replacement for nearly all of its functionality in v2. And even then we'll give plenty of notice (probably at least one major version/release) before it is completely removed.
There are no issues with mixed development between v1 and v2 web services. In fact, some of our new SharePoint work is done that way - mixing calls between v1 and v2.