1. Can I carry on with BLFS6.3 over LFS6.4?
Yes you can. If anything needs to be done differently it will be mentioned in the user notes link provided.
2. IF yes, Then how Can I get Graphical Environment working with min. no of packages to install.
As you might have noticed already, BLFS is a collection of software and unlike LFS you decide what is installed and what is not installed.
If you are looking for a specific piece of software you go to that chapter and look what the dependencies are. Some are required and some are optional, the latter is up to you if you need/want them. You also need to check if the dependencies have dependencies.
Example: If you need Tripwire-18.104.22.168 you need to install OpenSSL-0.9.8g first. OpenSSL has an optional dependency: bc-1.06 (it is up to you to [not] install it).
Hope this helps.