If I understand what you are trying to do correctly then creating a Custom Transform may be what you need. Most software packages have a set of default orientation axis that you use to move scene elements around. The typical transform orientations are global, view, normal, and local. Blender allows you to create a custom transform so you can transform an element along a user defined axis. To do this select one of the poly faces you are referring to as the "adjacent" poly. Press Ctrl+Alt+Space Bar. You should now see the transform widget align itself with the normal of the selected face. On the bottom left side of your workspace screen you will see the "Create Orientation" panel. You can now name your custom transform. Now when you select one of the "new" polys that you created you can use the custom transform to move it around on the same axis as your "adjacent" poly.

If I have misunderstood your question it might help to either attached a picture or better still post a link to your blend file on dropbox or similar.

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