Using USB Mini Adapter with Arduino That Already Has USB Onboard

I'm having trouble with USB connections with Arduino Nano 3.0 when powered with external power supply on a circuit board. Depending on the circut board configuration, sometimes they work, sometimes they don't. Very unpredicable. Apparently, this problem has plagued many others and was determined to be a floating 'Test' pin #25 on the FT232RL USB chip.

Gravitech has tied the test pin #25 to ground pin #24 of their Nanos. However, most other manufacturers adhere to the schematic on the Arduino site, which leaves the 'test' pin floating.

I have a large number of Nanos in my posession that have the floating pin per the Arduino spec. I am not skilled enough to short the pins on a surface mount chip, so I'd like to use the Arduino USB mini adapter, which has the test' pin tied to ground. This also allows me to get the larger "B" style USB connector, instead of a mini-usb connector.

My question is:

CAN I CONNECT THE FREE STANDING USB->TTL SERAL ADAPTER TO AN ARDUINO THAT ALREADY HAS USB ONBOARD?

IS THE CONNECTION AS SIMPLE AS CONNECTING TX to RX AND RX toTX?

IS THERE ANY MODIFICATION OF THE NANO REQUIRED?

Thanks for your guidence.