The plain diaper-pin looking thing is a Scottish regimental pin, I think, and is worn vertically. In a quick check, I could not find any documentation for that, but I believe I've seen it worn as part of a military uniform. Personally, I think they're too boring to bother with, even in silver.
Correct on both accounts in regards to the military-issue kilt pin.

I like them with day attire; one of my friends has a nice one in silver with a Celtic cross & a green stone in the center, which is rather nice.

Cheers,

Todd