1) what causes the beer to be sucked up the gas line? if i have CO2 tank pressurizing the keg at all times can this happen?
This commonly happens in two different situations. First is when you hook up a fully carbonated keg that is already at higher pressure than your setting on your regulator. For this reason you should always release pressure off the keg just before hooking the gas up.
The other situation can arise when folks don't understand how to balance a keg system. These are the folks that keep the pressure high to keep carbonation where it should be and then lower the pressure for dispensing. This creates the same effect as I mentioned above.
In any case, you should always have a check valve in line somewhere to keep the beer from reaching the regulator.
Should i be disconnecting from the out port on the keg when not in use? if i do leave it on all the time with beer in the line, will the beer in the line be ok
You don't need to disconnect the line when not in use unless something in the tap line leaks. In that case you need to deal with that situation asap rather than just disconnecting it. The beer in the line will be just fine assuming it is inside the kegerator. Once you have your taps installed, you will always have beer in those lines too.
Wayne