SP - since we are in the same operation, the following may be of use to you and your colleagues:-
Jet engines are designed to run with permanent, intense flames inside them. These flames can sometimes be seen inside an engine that is running normally from astern of the engine at night. On occasions, NORMALLY during engine start, over-fuelling can sometimes result in what is called a tail-pipe fire. Sometimes clouds of white smoke appear as the fuel is semi-burnt. The procedure to deal with this is to keep the engine running so that the excess fuel can be burnt and blown safely out of the exhaust. PLEASE ONLY ADVISE THE F/DECK if you see this. An evacuation initiated by you would result in the engine being prematurely shut down with possible disastrous consequences, the probability being that the burning fuel will impinge on part of the structure. Evacuation into running engines or from a moving aircraft (you didn't mean that, I trust??) causes tears. You cater for the time when there IS an engine fire during start-up by talking to the flight-deck if they haven't already contacted you. The Captain will decide (guided by you, of course) whether to evacuate the aircraft
Our company lay down GUIDANCE for c/crew as to what is to be considered 'catastrophic' (FCO4403 and SEP 737 11.2). There should be VERY FEW situations where c/c should ever intitiate evacuation on their own, and every option to contact the f/deck should be explored. Just about (NOT exhaustive, and disregarding post-impact) the only situation where I can see a c/c initiated evacuation as justified would be SIGNIFICANT fire with the aircraft stationary, with smoke or flames entering the cabin. As I say, though, not exhaustive.
You had better raise this with SEP on your visit next year? It seems there are a few gaps.