Hello, I am new to these forums, but a competent biker. I'm a student residing in Tokyo and I just recently bought a single-gear bike, it's really fun. Today I went to the store and bought a 17-Tooth cog(? If that's what it's called, still new to the terms.), fixed gear, it also came with a lock ring, but there is where the problem is... As soon as I could, I brought the bike to the store and I asked if they could transform it from a single-gear to a fixed gear, I gave them cog(?), they said yes. Couple minutes later, they said that the lock-ring that came with the cog(...) doesn't fit; my question there, is how could it be??? They asked me if I wanted to proceed with the installation and I said yes, they warned me that if I pedal backwards I can destroy my hub(?). Because I don't exactly plan on dying, and I heard that riding without a lockring is dangerous(I have brakes so I should be fine...right?) I want to know what exactly will happen if I pedal backwards while I'm on the road, what happens??? And my last question is, how can I put a lock-ring on it that actually fits(I'm bewildered about the ring not fitting), because I want to be cool and safer with 3 alternative brakes.

Thanks guys,
Meltedsnake~