I can answer some, although not all, of your questions.
1.) For beveled edges, I scrape mine with a hobby knife. I think I'd like to see a example picture for what you mean though.
2.) For vehicles, you want a thickness about equal to a credit card, maybe a hair thicker for rivets. Something
like this, though if you are making vehicles you should instead get a
assortment pack like this.
3.) For big, chunky rivets, get 3/32 rod or even 1/8th for very large rivets, like on a superheavy.
4.) I tend to buy online from either
Hobbylink,
Tower Hobbies, or ebay, depending who has the best prices for what I need.
5.) Again, need an example image to see that beveled look.
And, as a bonus:
I think making rivets out of rods doesn't look particularly good.
Here is a tutorial that shows you how to make way better looking rivets right now for free with things you almost certainly already have somewhere around the house (an old speaker grille or fan cover for a
PC, or headphones, or something) and scrap sprue.