Usb serial communication with raspberry pi and software serial at the same time

Hello, i want to connect arduino uno to my raspberry pi , i found a good tutorial here Dr. Monk's DIY Electronics Blog: Raspberry Pi and Arduino and i want to try it.

But before this , i want to know something... I have 2 sensors Humidity and Temperature Sensor - DHT20 - SEN-18364 - SparkFun Electronics? and the reason i want to make the 2 boards to communicate is for sharing the sensors measurements.

I have 2 software serial devices working right now, a gsm shield (pins 2,3) and an lcd (pin 4) , so can i send serial commands from the usb the same time ?

Finally I will use an external power supply of 12V for my arduino , connecting the 2 boards requires a usb cable , what if i use only 2 wires (green white) , the data wires, and cutting red/black (power) wires of the usb cable , it will work ?

Thanks !

you dont want to disconnect ground, thats bad mk? ("ground" can be at different levels on different devices, hook them together to make sure they are all at the same level)

yes you are right i said it because i don't want to power arduino from external power at the same time with a usb cable.. i know it automatically switches but... im new and im afraid.... GND pin of arduino is also required ? i thought external power cable should be enough for arduino...