Hello,
According to the IPS documentation, the requirement of 8 characters minimum for the password can be found on all versions all the way back to IPS version 4.x:
Step 2
You are prompted to change the default password.
Passwords must be at least eight characters long and be strong, that is, not be a dictionary word.
http://tinyurl.com/2cg7g63
Here is link to all the "Install and Upgrade Guides" for IPS versions from 4.x to the latest 7.x:
http://tinyurl.com/2444jwt
The 8 character minimum for the password is mentioned under "Initializing the sensor" in all versions.
Hope that helps!