Heart is considered a muscle meat but it is quite rich so I wouldn't serve the entire meal of heart I'd mix with regular meat.
I see you are in Toronto, you need to join a group of FB called heronview raw and natural.
Lori is a breeder and purveyor of raw natural animal foods. She has an incredible knowledge of holistic remedies as well as a massive amount of info for new raw feeders.
https://www.facebook.com/groups/59979896589/
She is located in very North Whitby, but it's really really worth a phone call and a meeting with her.
Heronview Raw and Natural Pet Food