Arduino Hardware serial pins with RS485
1 - Can i use Hardware serial pins with RS485 for 40ft distance, i tested code and it does work but i couldn't test that for 40ft. So not sure if it works or not.


Yes.


  2 - Is it good idea to use hardware serial pins for my project i explained, as i see plus and minus over internet and making me confuse. I see majority people recommend Hardware pins though.


Yes. SoftwareSerial is awful.


  3 - Is there limitation connecting multiple arduino nano with this approach, i only did this with 2 so far.


Yes. The MAX485 and similar chips can only handle up to about 30 devices on one bus. The exact number is in the datasheet for the chip.