I'm using the on camera flash to trigger the remote flash.
If I dial down the flash compensation on the camera will that affect the remote flash? Will that help cancel out the on camera flash?
The optical trigger needs a certain amount of light for it to work...so I would suppose that if you turned the flash down too far, you might have trouble getting the remote to fire.
Most on-camera flashes use a pre-flash, which usually triggers the remote prematurely. it seems to be working for you, so either you don't have a preflash or the optical trigger is smart enough to ignore the preflash.
Actually, using the on-camera flash for fill, is a pretty good idea.
When you are taking a portrait, you want to be concerned about the light ratio on the subjects face. That is the ratio of light on the bright side compared to the light on the dark side. Even lighting would be 1:1, a one stop difference would be 1:2 etc.
This is where it's very handy to have a flash meter, so that you can measure either side and know what ratio you are getting.
You off camera light is probably going to be your Main (or Key) light. This is the light that will light the face and also cast some shadows. The fill light will add light to the whole scene, both the shadow side and the bright side of the face. By adjusting the strength of the lights, you can alter the ratio on the subject's face.
We usually say that a higher ratio is more 'dramatic', where parts of the model's face are in dark shadow.
It's up to you, to decide which ratio is going to be best for your subject and the type of portrait that you are going for.
Another good way to use fill light, is to get a reflector and put it opposite the main light. This will reflect some light back onto the subject. The closer the reflector, the more light you will get back on the subject.
As for learning the lighting patterns, I'm sure there are some great books and websites etc. Google is your friend. I learned a lot of what I know, from some portrait lighting classes I took. That might be something to look into.