So I have a map where the players can get hydralisks, and I'd like them to be able to convert them to lurkers. If you try, it says you don't have enough supply. Is there a way to change this (other than by adding overlords)?

I saw a tutorial on the subject but it really doesn't make any sense to me. Why would creating units (other than overlords/depots/pylons) change the supply? What's up with players in the 100s and 200s? Are they talking about some SC mod or something?

It uses extended players (SCMD (I think) or SF to place them) that creates a buffer overflow (similar to EUDs, but less controlled) to edit certain values--in this case, supply values.

You can use scmdraft to place extended players. Out of curiosity, what other units for extended players create these buffer overflows?

A simple and easy way would be to have a Queen that can brood, and then brood the hydralisk. Remove one, center the location on the other brood and remove and create a lurker at that location.
You may want to make the location you center on the broodling rather large, however, because I assume the hydralisks will be in groups, and there might be a unit unplaceable message.

http://www.staredit.net/topic/3487/ It doesn't list them all, but it has like 3.

Mostly it just takes a lot of testing to find the cool effects you can get using them, testing which hasn't been done because it is so random and tedious that it's annoying to do, and you often miss the effects anyway since they come in such a wide range.

Another thing is that, are they like EUD's? By that I mean that, do they change with new patches?

I don't know for sure, but by the nature of them, I would guess so.

Lucky for us, starcraft is pretty much patched out.

Yeah, any buffer overflow might change after every patch. Also any buffer overflow can have unnoticed side effects which may screw up things later in the game. All the extended players should cause buffer overflow, however only some of them have known effects.