you're allways using 1 pixel for the rivets. (they're too big anyways, compared to real scale...) if you're working on a 1 k texture sheet and you're rescaling it later everything is fine. keep in mind that you probably don't have any details on a 256k texture. (at least not very visible and blurred out at all)
I've never heard that pure black or white is used as an alpha channel in the rgb channel itself, to be honest it doesn't make any sense to me. the alpha channel is usually a seperate channel, additional to r-g-b. I'd be very suprised if you can't use pure black/white in the diffuse texture itself.