... is now causing to show about the last 4 or 5 collums black when i turn the potentiometer ...
You very well may have a defective LCD.
could it be that this lcd is not Hitachi HD44780 Compatible?
It is not likely.
how does non-Hitachi HD44780 LCD's react to this libary?
They typically work properly
Once again I will recommend that you disconnect all of your wires and start again and carefully follow the tutorial at Arduino Tutorial - connecting a parallel LCD.
Before you get to the point where you connect the six wires between the LCD and the Arduino you should see a single row of boxes on the top line of your display (the LCD in the tutorial is upside down). If you don't get that display then there is no sense going any further.
Don