1, its not that they've never crossed with goldens, they're just a line of shrimp that makes pure reds. This name is given now to any shrimp people want to make sounds special. Basically think of it as a Brand Name shrimp, as opposed to a home made shrimp. They would no longer be PRL (pure line) if mixed with blacks, or even some other brands of Prl.
2 yes eventually it'll be mostly reds.
3. Yeah keep culling and youll get there. If you think you have pure lines and ever wanted to test them for line integrity, simply take one of your pure line shrimp females and put with a golden male. If any offspring are goldens, you do not have a pure line (since the golden gene is recessive).