On a side note, if what I'm reading the DMG and other sources correctly, Holy and Unholy aren't actually damage types; it's mostly just applied to untyped damage that then won't injure good/evil creatures. So while it's an attractive idea, I think I might stick with the Negative/Positive energy for the moment, because this is complicated enough already.
I was originally thinking that demons and devils would have the first combination, but if I had to differentiate I'd probably make the other Fel-energy combination Cold/Acid/Negative energy.
That way the two are similar, but you still have:
Hot Hell= Chaos/Fire
Cold Hell= Order/Ice
Hmm....I'll have to think some more about how this can be applied, but overall I think it actually just sounds more complex than it really is. It shouldn't require any more calculation than a normal energy-based attack would. The rule I had put was basically "the energy type is whatever type you are least-resistant too".Anyway, instead of requiring immunity to overcome and getting knee deep in calculations, make it so that a warlock can apply one or two types of his choice, but not all three.
This will:
1. Not entirely break the suspension of disbelief even for those of us with an even lower threshold.
2. Simplify calculations.
3. Put in the element of combat tactics.
So if you had Resistance 10 against all energy types, it would help you more than immunity against any 1 type.
Also, just in case any one was worried, I certainly wouldn't make all the untyped damage sources this complicated. The Warlock was just a good example of something that had lots of OTHER options.