How do I get them? Is it for every weapon!?
Do I really have to make a reboot?

Why does the Nano suit reboot delete all the kills and just not the rank?!

You don't need to reboot. You just have to reach maximum level (50 and then get some more exp until it says Maximum Level Reached). After that, the Cell skins are unlocked for three weapons already: Scar, Feline, and the Hammer.

Edit:
And oh just one more thing: I did a nanosuit reboot before the 1.4 patch. But I'll tell you, I used the cell skins even before I rebooted. I stayed at the Maximum Level for about three days while using them. Unless it was really a glitch by then, then I'll stick to my post.

You don't need to reboot. You just have to reach maximum level (50 and then get some more exp until it says Maximum Level Reached). After that, the Cell skins are unlocked for three weapons already: Scar, Feline, and the Hammer.

Edit:
And oh just one more thing: I did a nanosuit reboot before the 1.4 patch. But I'll tell you, I used the cell skins even before I rebooted. I stayed at the Maximum Level for about three days while using them. Unless it was really a glitch by then, then I'll stick to my post.

I saw I video with a guy telling us that he already had the cell skins before reboot,

yes there are cell skins before reboot, but just for 3 weapons, I believe if you want to have cell skin for guns like scarab or grendel you would need to lvl up to 50 a 2nd time