Yes you can. I would wait until I got down to a 1/4 tank and turn the valve on.
When your tank gets full you can leave it open and you shouldn't have a problem. I did not. But it is so easy to turn the valve off sometimes I would then just turn it back open when at a quarter tank.
The system is sealed and should not cause any problems but refer to above.
Every once in a while I would have to turn off the ignition and restart to correct fuel gauge. Can't remember the exact issue but that always solved the problem.
I added nothing but some do. I do not think it is necessary.
Overall the gravity feed works perfectly and I used this on my 08 and 2011 duramax.
I purchased the 50 gallon with a tool box on top. It was still below my bed rails so if I wanted to use a cover I could.
All and all very happy with this purchase.