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.
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Although I am a VMware employee I contribute to VMware Communities voluntarily (ie. not in any official capacity)
VMware Training & Certification blog
