I've read and heard that -6db is a good level to record at, or near that area. Also would it be better to use my faders in the pro tools mixer to achieve these levels, or to use the gain knobs on my Tascam US-1800 to adjust the levels that are recorded.
These are different points in the chain and would be adjusted for different reasons.
Starting a session you'd leave the PT faders at unity and forget about them.
Use the gain knob on your interface to get a signal that's loud enough for background noise/interference not to be heard, but low enough that your converters aren't clipped.
There
is a limit to what they can take.
One all the recording is done, the ProTools faders are there for mixing.
If you recorded too hot or too low the PT faders
will not fix the problem.
At this stage you're mixing several healthy tracks together so the Master meters are your point of reference.
Again, make sure the sum of the parts doesn't clip the master track. Leave some room!
Also, don't jump ahead.
There's no point having heavy limiters and compressors on while you're mixing.
Keep your master track empty and use it as an overall mix volume reference.
Once you're completely finished mixing you'd take a look at mastering although I guess it does no harm to test a limiter on the master once in a while just to see how your mix would play out with it, especially if you're inexperience with that kind of thing like me.
Hope that helps.