Heart is considered category "meat", not "organ", when formulating homemade raw diets.
"Organs" are usually "secreting" organs, such as the gizzard and kidneys.
So using heart in your mix as a MEAT consideration is fine. Let's say your mix was 45% heart, 45% turkey thigh, 5% liver - you'd need to add in some organs to really complete things, so 5% of gizzard, kidneys, whatever you can get your hands on. But the heart is not going to count towards the ideal ~10% of "organ" meat in an idealized 80-10-10 raw-food diet. That's 80% meat, 10% bone, and 10% organs, broken down to 5% liver and 5% other organs. The bone content is very important to remember too.
And yes, you need to supplement any homemade raw diet, even if the diet is only supplemental/not their entire diet. :} Canned foods and kibbles are formulated to have the vitamins and nutrients cats need, but raw diets do not inherently have ALL the taurine and nutrients - that's why kitties can't just be fed raw chicken bits and call it a day - the proper nutrients aren't there. If you won't be giving any bone to your kitties in your homemade raw meals, you may want to check and see if the supplement you're using has calcium. For example, Alnutrin, the brand I use, makes both a "meat and bone" supplement (for mixes that already have bone) and a "meat only" supplement with added eggshell calcium (for mixes that do not have bone).
Take Heart, But Not Too Much - Feline Nutrition
This is in regards to chicken hearts, but I imagine the principle is the same for veal heart. I don't THINK you're overfeeding heart, but I generally use pre-ground raw mixes from Hare Today these days, so I'm not entirely sure.