I went ahead and separated out the tissues so that only obsidian dwarves are immune to flame and only ice dwarves are immune to cold. This fixed the problem with the aspid dwarves' venom, so I switched the doom dwarves back to aspid dwarves.

To install, copy these files into the raw folder of your genesis mod (version 3.18, graphical). It seems to not break save compatibility, but I'd make a backup of the save folder before installing this into an active save.

(I tested it by gening a world under default 3.18 raws and starting a fort, saving, exiting the game, and copying this patch into the save's raw folder. No error log was generated and it didn't crash the game).