The party mage has a force 3 fire elemental on hand and orders it to attack a genericorp goon. The Fire elemental charges in and attacks, doing Str/2 stun damage as an unarmed attack and gets +4 fire damage on that. Now fire damage follows the rules in the combat section, meaning it does physical damage.

Now we have an attack that depending on your reading could be doing two types of damage that are resolved radically differently. What damage is done? The two options as far as I can tell are that either:

1. The attack does two different types of damage which are somehow resisted separatelty
   or
2. The entire attack is converted into energy damage

Secondly, all fire elementals have the engulf power and the energy aura power. The engulf power deals damage equal to the creature's magic and is resolved as a melee attack. The energy aura power adds +4 to the DV of mele attacks made by the creature. Does that mean that a force 3 fire elemental is doing a base of 7 damage (3 magic +4 for energy aura) on its engulf attacks? Isn't this a lot?

Thirdly, is there really anything to stop someone from summoning a different-energy variant of one of the elementals? IE: Say I'm wanting the offensive abilities of a fire elemental during a lightning storm. Could I summon a variant of the fire spirit which uses electricity on all of its attacks rather than fire?

the "storm spirit" thing is perfectly doable... you will need at least force 6, but if you look you will notice that as optional powers, air spirits can choose elemental attack and elemental aura. now, technically, it doesn't say this, but i would personally rule they cannot choose fire

as far as the other stuff... i would presume it's all physical fire damage, personally. SR4 seems to have done away with half and half attacks, such as the old shock gloves.

and yes, 7 DV is quite a bit, but keep in mind this is a fire spirit engulfing someone in flames... it's kinda like getting hit by a flamethrower, except the flame is sentient. naturally, it's going to do quite a bit of damage.

i would consider your desert heat/haze spirit to be a fire spirit, personally

i dunno, maybe if you had a desert shaman (unable to summon fire spirits) i would allow it...