[Anaraxes]

I think the recommendation to have Allies built by the GM is to avoid player abuse by crafting an Ally that's an exact complement to the main PC's skill set and Advantages, with no overlap or duplication, and precisely selected synergies. That's less of a concern when it comes to picking the exact ST or IQ of a fire elemental, so that level of choice could be left in the hands of the player.

(I'm assuming "fire elementals" have a fairly well defined type, probably a template, in the setting, so just that phrase gives you a good idea of what it can do. If any sort of creature with arbitrary magical powers can be summoned on a whim, then you might need to reinsert the GM intervention. Though such a "Modular Ally" would already be hugely powerful.)