I believe that's commonly used to kill animals that are currently annoying your fort. Save and quit, modify the raw in the save file with [NO_BREATH] (or whatever it is), reload, and watch the creatures suffocate. No idea what happens if they don't have [NOEAT] but no guts.
You can find a part in the creature raws, [CREATURE_TILE:'d'][COLOR:7:0:0] for example, to control the symbol used to display the creature, and the color. If, for example, you want ALL slimes to have the same symbol but different colors for the castes. You leave the CREATURE_TILE part in the main entry, and delete COLOR from there. In each caste section put the COLOR part. Use the page from the wiki on color,
http://dwarffortresswiki.org/index.php/DF2012:Color , to figure out what to change the numbers to. Also, if you have colors set under TL_COLOR_MODIFIER for use in descriptions, use that page to set colors the game recognizes.
One other thing, don't use just the arena for combat testing. The quality of weapons affects things a lot. I had a creature I made that was unkillable by steel weapons in grandmaster hands, with groups of ten to twenty dwarves and the creature was unharmed when the dwarves were all dead (other than it's eyes). Yet when I ran into it in a real game it was quickly beheaded with a masterwork steel axe.