is it true to you should stick to the brand of cog that you have already installed on your hub due to minor threading differences?

my superstar was only $32. new. =D

Yeah phil wood cogs are passable... I would tell you what is better but I have to fill my bottom bracket with fresh caviar... tootles!

If your hill is huge and you have a flip flop hub you could just flip the wheel before riding back from work. If your commute is short, then it may not be worth it, but it doesn't take much time to switch.
I use a Surly cog to get either a 46/17 or 46/19. The difference helps if I'm carrying a bunch of stuff, if it's super windy or if I'll be going up some big hills. If everything is normal the I can get more speed out of the 17t.
Sounds like you have a nice rig, but don't sweat the cog brand too much. Surly or Dura Ace is what most folks use. I use Surly because they come in larger sizes.
Get a lock ring. They are essentially commodity parts, so don't get hung up about what kind. Your nice hubs will last longer for it.
Since you seem new to some of this, don't forget to grease things up before putting on a cog or lockring. Riding them will tighten them. Grease will make your hub last longer and make the cog easier to get off.