Changing cogs in cassettes and other questions
 

can i replace worn or bent cogs in cassettes?

im building a fixed and need to choose between 3/32 and 1/8 for chain and cog sizes. What are some of the benefits and drawbacks of each?

Cogs or groups of cogs if mounted on carriers can be replaced.

can i replace worn or bent cogs in cassettes?

Yes, as Sydney said many cassettes can be taken apart by removing the holding screws or rivets that keep the groups of cogs together. However, individual cogs are priced so high that rebuilding a cassette by replacing three or more worn cogs often costs more than an entire new cassette.

im building a fixed and need to choose between 3/32 and 1/8 for chain and cog sizes. What are some of the benefits and drawbacks of each?

3/32" chain and cogs are much more common. Otherwise, there is no advantage to either. The theory is 1/8" chain is stronger but that's of no importance. If 3/32" chain handles the rigors and torque of extremely low gears in MTB use, a fixed gear will never hurt it no matter who's riding it.

Can't comment on your fixie question but, as others point out, cassettes can be dismantled and rebuilt.
FWIW, I did this about two weeks ago, per instructions on Sheldon Brown's site, replacing the 28T large cog with a 34 to give me a lower bottom end for climbing hills on a loaded touring bike.

This was an 8-speed Shimano cassette. On the large cog side of the cassette, I had to grind off the ends of three LONG rivets, taking care not to damage the cog, and slide the rivets out to let the cogs and spacers fall away from each other.

The 34T cog was GIVEN to me by the LBS - taken right out of their "box of old parts."
I suspect you've got just such a box waiting for you at an LBS close to you. Make a list of what you need and perhaps find out what brand of beer the mechs like. :)