Start thinking as a musician who plays bass, guitar, sax, keys, drums, trumpet, violin, oboe, flute, timpani, tuba, tambourine, harpsichord...and writes originals. That's when all of this begins to make sense. How and why the instruments interact the way that they do. Getting back to your original question, major and minor pentatonic scales can and have been used in the same bass line, but probably weren't necessarily thought of as such. Possibly the writer was using fragmented portions of modes linked together. Think of playing a C# as opposed to a D flat. It usually depends on what key you're in, but the actual note played is the same. Now go one step further, imagine the composer had some novel idea about having instruments playing in different keys simultaneously. (I think Zappa did that a lot.) If you're learning the bass line by ear and ask the piano player what note he played, he'd tell you one thing, the bari sax player would call him a liar, soon chairs would be flying...mayhem. If they pointed to where you should put your fingers on the fretboard, they'd both be right. Sorry to go off on a tangent.