I'm planning to power the pin 27 of my arduino directly via 5V adapter. I have read many forum posts where it says it is safe to do so, if the voltage provided is not more than 5.5V. Also, can I use the USB port simultaneously to the arduino nano clone, if I connect a diode (IN4007 for example), wherein the anode is connected to the negative end of the 5V adapter and the and cathode is connected to 5V pin (27 ) of nano board ?
Reference post:usb - Prevent 5V output on Arduino Nano 5V pin - Arduino Stack Exchange
Or should I connect a diode (IN4007) between vin of arduino and 5V pin of arduino , as suggested here
https://forum.arduino.cc/index.php?topic=271158.0
I don't think you need to add anything for a Nano. The USB 5V line is already connected to the 5V pin through a Schottky diode. So if both sources are active, the external 5V source will always supply the current to the Nano. The existing diode will be reverse biased, so no current will flow through it in either direction other than a little leakage current.
It might be different if you were applying 7V or higher to the Nano's Vin pin, but you aren't doing that.
lennarddevoogd:
but you can apply max 24v on the Vin pin of nano but 5v or slightly higher is recommended
Sorry, but you really do not want to use the "Vin" pin of a Nano to power it. Stick to 5 V to the "5V" pin and certainly do not power "Vin" with more than 12 V. There is no 5 V problem at all with the Nano due to the diode built in, nothing else needed. 
Well, actually, there is a small problem - the diode will lose some voltage if you power it via the USB connector, so that is best used only for programming.