So as I installed plasma desktop it obviously took over and now instead of desired login screen(picture) I see some KDE lightdm theme..would it be possible I could have the default Ubuntu 12.04 login screen(on picture-i can't post it bc of reputation but you know the one with grid and all)?
You need to install
unity-greeter if it is missing.
sudo nano /etc/lightdm/lightdm.conf
and change the line
Save and exit and either restart
sudo service lightdm restart or reboot.