So, since Passionpit is now currently a fusion, I think this will be helpful for future ones.
Fusions are two cats/dogs fused together to make a creature but with added features and twice as large. A forced fusions is where someone shatters two other gems and sticks them together to form a cluster. A normal fusion is a fusion that is normally made by a dance, and they need to be in contact.
None at the moment.