But you, the clerk, you can not choose what will happen. By the text of Planar Ally:
This entity sends a celestial, elemental, or faithful demon to help you, making the creature appear in an unoccupied space within reach. If you know the name of a specific creature, you can pronounce that name when you cast that spell to ask that creature, though could get a different creature anyway (Choice of DM).
There is no limitation to the CR of the creature, of any nature whatsoever. The DM will most likely try to fit the scenario and your relationship with the entity of another word. Note that the creature summoned …
is under no constraint behave in a particular way. You can have the creature perform a service in exchange for a payment, but do not have to do this.
… although it is sent mainly help you:
This entity sends a celestial, an elemental or a demon loyal to it help you
Nevertheless, you should pray your DM right after your cleric finishes praying for a planar ally.