You should always take time to assess the situation no matter what is happening. Don't just start shouting evacuate until you have a grasp of what is happening.
You mention that you would tell the pax to evacuate as soon as you stop. Well that is just madness. For a start the fire warning might just be a faulty fire detection system and as such there actually is no fire! Even if there is a real fire then you will most likely still have a running engine on the side that doesn't have the fire! You tell people to evacuate then they are going to be sucked into the engine/run into a spinning prop.
You don't mention if this is multicrew or not. If so then one of you will be doing the engine fire checklist whilst the other is trying to get more information (this can come from anywhere! Cabin crew who can see the engine, ATC, other aircraft taxing near you etc) As for whether to turn the aircraft or not there is no right answer. Most people I have spoken to are split down the middle. Some say turn, some say don't. I personally agree with the KISS philosophy! (Keep It Simple Stupid!) Turning whilst stopping is just adding to your work load in an already busy situation. I say stop the a/c ASAP, and assess. Then act on the information.
Once it has been established that the fire isn't contained then you head off into the evacuation checklist. Some of the actions will most likely have been done (like pressing the relevant fire buttons and agents) but the most important thing is that the OTHER engine, as well as the engine on fire (it may well still be spinning even with a fire) will both be shutdown and will allow a safe evacuation.
As always every situation is different but the phases of how you deal with it should always be the same. In my opinion there is ALWAYS time to just pause for a couple of seconds to assess the situation and take in what information you can. Don't rush into an action that may well be wrong.