Is there any resource that lets you know what unit graphic can be swaped with another unit graphic via the EUD Editor?

Experiment and experience

It relies heavily on how much you want with new graphic.

If it is unit...

• Almost every graphics can move around. (check out all script checkbox)
• To be able to attack, vulture or wraith iscript enables to attack even with powerups or doodad image.
• If you want nice attack animations, it becomes headache.
• If it is building and needs research animation, uses Ion Cannon iscript.

It it is weapon graphic,

• Only iscript of weapon hit image can deals damage, so you have a little choice.

Thanks man. Are there any "pro tips" that can take away a lot of the trial and error? Eg. So and so graphic will always crash, etc.

I'm specifically after if anything can be the dark swarm sprite.

You can use an iscript editing tool like PyICE to look at unit iscripts. That will tell you what animations a unit has and what frames it plays during them. Playing a non-existent animation or frames is generally what causes a crash.

I wrote a tutorial for basic iscript here: http://www.staredit.net/topic/17402/#6

The editing portion won't be useful to you, but the beginning part should explain how to read iscript.