Anyways the new theory.

Notice how jiraiya saw that all those bodies were ninja he defeated in his past but how could pein have known about them ....

What if he read jiriaya's book about the ninja and found them that's what i use to think... But what if pein his rinnegan has the power to over take someone's body and he used those weird metals to suppress their chakra so they have no way of resurfacing to their usual self and instead are now his bodies...

Naruto 381 page 03 | narutobase http://www.narutobase.net/Gallery/Na..._pg03.png.html

Notice how peins rinnegan just appeared behind jiraiya as if pein saw what was going on and started to take control right when his eyes appeared jiraiya said "shit my chakra is going crazy i have no control"

Maybe his ability works in conjunction with these weird metals why else would there be that much on there faces what purpose to they serve.

I think pein somehow found these bodies but didn't kill them he merely took over but not like a soul transfer he just transfered part of his skills around these 6 bodies and took over using those metals that suppress chakra he then uses some ability to fully control the body thus making it his...

Naruto 381 page 09 | narutobase http://www.narutobase.net/Gallery/Na..._pg09.png.html
"you will travel and write a book " a book but why"......

perhaps pein somehow found these bodies through the book and made them his with the way i mentioned above...

althought i'm not so sure about it.