The heart spawned from the pizza box will attempt to be a type that has not been spawned yet. The same type of heart cannot spawn again until all of them have appeared.
Having multiple copies of Reheated Pizza will spawn additional pizza boxes in the starting room, based on the amount of copies.
The costume granted by Reheated Pizza is based off of Peppino from the 2023 indie game Pizza Tower.
With Fiend, Tainted Fiend, and Golem, they will have unique costumes that references the characters The Noise, Fake Peppino, and Gerome respectively.
According to the developers, this item originally wasn't supposed to be a Pizza Tower reference, but the costume was added later down the line because they thought it was a funny idea.