As a moderator, I've moved the thread to the Education Services area - as the thread isn't about vSphere itself.
Regarding your question, yes I think you can mix the 6.x versions - based on what it says on this page: VMware Certified Instructor
The real key is getting a sponsor VATC, as mentioned on the page I've linked to - without sponsorship you could have attended every course and hold every certification but still not meet the VCI criteria.
Some of this depends on the region. At this point, all regions should be 6.5. Keep in mind that the attendance of the ICM isn't just for learning technical info but also presentation/soft skills. Anyone can know the technical but the important thing is how to get that information across to students. We do recommend that at least ONE of the sessions be in person.
And the VATC requirement is a hard requirement as well as the other items required. You can find full details here: http://www.vciportal.com or send me a note as I'm one of the leads on VCI certification.