Oooh boy.
 
PYRO:  
A common misconception is that the Pyro thinks he’s spreading joy. The comics show us this is not the case; he’s a pyromaniac. When he’s around fire, it seems his world becomes colorful and lively; when not-so-near fire, it’s dull and gray, and people look like blocks. His grasp of reality is broken at all times; there is no break for him, he has no understanding of the damage he’s causing, and he doesn’t seem to really care.
 
Pinkie Pie is a hyperactive, energetic girl who loves to spread joy. She is known for organizing parties, hiding random objects around town, and using a party cannon as well as other weird gadgets. While she is random, there seems to be a method to her madness.
 
Pinkie Pyro throws that away and focuses on Pyroland and Pyroland alone, with a complete and utter disregard of any other aspect of either the Pyro OR Pinkie Pie. People argue that Pinkie fits for her psychosis in ‘Party of One’ - problem is, Pinkie Pie was utterly depressed, in contrast with the joyous nature of the Pyro. Her psychosis also caused her hallucinations to speak to her words of treachery and deceit; there was nothing happy about her break down. Literally, using this logic, Pinkie Pyro would be a mopey, psychotically depressed pyromaniac setting everything on fire in a state of horrific, rejection-based grief.
 
Unless you’re going to use Cupcakes as a source - but if you’re using fan fiction as a source for anything, all bets are off and all match-ups work.