Is there a way by which I can choose if I want to login to ubuntu(unity) or ubuntu classic at grub loading it self. I have autologin enabled and I want to choose the session at the starting. Or is there a way where I can change the script which is needed to load kernel and run other scripts which invokes gdm?
As far as I know, Ubuntu runlevels 2-5 are the same. Just modify runlevel 3 or 4 to achieve your needs. In GRUB, add a new line to it with your newly created runlevel.
In /etc directory, there are rcX.d directories, which controls services on different runlevels. As I see, rc3.d contains the same links as rc2.d to services in /etc/init.d, so create a new GRUB entry and put "3" at the and of the line without quotes (this will start your system on runlevel 3). Start this, set up a different graphical environment. After restart, this entry should meet your needs, I think.