I'm pretty sure that I can use 'past tense' where we can use 'past perfect'. The only difference that I feel is if speaker is willing to emphasize the fact something had happened before another happened. Here are examples.
They started to work on this project, and about a month after that, I started.
They had started to work on this project, and about a month after that, I started.
They started to work on this project about a month before I started.
They had started to work on this project about a month before I started.
I feel all of them are available and natural, I prefer #2,4 though. Am I right?