Regular Chaos Serpents aren't used very often anyway and would still fit (They breathe fire, for crying out loud). Adding Volcano Serpents would just be one more monster that isn't seen often, when other monsters still work fine. Hell, even the Maulotaur would still work.
Is the map on the SVN? If so, point me to the area and I'll give a better opinion of which monster would work best.
Also I tried using the new texture with the lavafall behind the disciple but it's not properly animated, so you guys know.
Post a bug report, so we can better track it.