I've been checking around a bit, from cites citing research, but I haven't myself read the actual research.
Cardiotoxicity seems to be lower in MDMA than its rumored to be, with moderate recreational use. Though slightly higher than amphetamines. Given that you are considered healthy enough to use Adderall (on a daily basis), it is unlikely that MDMA will cause damage to your hearth. If your heart is well enough that you think you can do moderate exercise for a few hours, it should also be good healthy enough to MDMA. MDMA increases muscle tension in your body, which is why users clench their jaws.
But with low and sensible doses, and a sensible physical activity when you roll, this isn't unbearable strain for most hearts. MDMA isn't like being tazered, it just highlightes the overall body activity, so you will get warm.
The heat is the cause of risk of neurotoxicity, not the MDMA itself. To avoid the possible negative effects this can give after rolling (prolonged mdma hangover), it is wise for all user to not increase heat with excessive physical activity (like dancing all night long).
In your case this also adds as a precaution for cardiotoxicity, as dancing would be something that also add additional strain to your heart.
If you are anxious about how your heart will react to MDMA, wait until you are used to Adderall. Recent research show that there is a minimal risk of cardiovascular desease in adults using prescribed stimulants. The connection between stimulants and cardiac damage/desease appears to appear together with heavy use over several days (binges), or in combination with other drugs such that the combination increases toxicity.
You wish to combine MDMA with heroin.
I'm not too familiar with opiates, but it seems like herion brakes down to morphine when taken orally, which again is metabolised in the liver, but by different enzymes than the MDMA. By that it seems like it's no critical direct drug-drug toxicity, but some rats once experienced more potency from morphine after first having rolled on molly. I don't know the details.
Morphine would probably dampen the negative side effects such as muscular tension, but would probably also dampen the roll. If you take morphine so it is effective when MDMA peaks, you may also sell yourself short of the psychedelic experience. MDMA expands your mind, and is a wonderful helper for introspection, a state which monks need years upon years with meditation just to get a glimpse of it.
I'm going to make an assumption now; thats not going to happen with heroin. Morphine makes you feel good, it does not expand your mind and help you to get the possible benefits from mdma. It's probably going to feel good, one way ornthe other, but if you combine them you could also just end up disappointed. A big part of the MDMA euphoric extacy is also increased sense perception. The sensation of skin, fabrics, fur. Morphine will do it's job, and reduce this.
Also, as a general rule; the more drugs in the mix, the higher the risk. And if you mix, my suggestion would be to at least mix it yourself. If it is in a pill you have less controll over the relationship between mdma and molly, and I would strongly recommend testing it.
Personally extacy (pills) freaks me out a little, but I'm lucky enough to have access to powder. Even if I trust my source, I always test it. And I use a mg scale that is accurate, so I know of it is 119 mg or 124 mg.
I have ADHD, and in my experience I get super energetic of MDMA if I'm not also taking my ADHD meds. But I started with MDMA alone, and that would also be my suggestion for everybody else.
Rather than having one new drug in your system, not exactly knowing how you will react to it, you will also not know how MDMA and heroin will react together, making it all more unpredictable.
If you have never tried psychedelics before, you are missing out on something important. With the right setting, MDMA can make you feel so good that it's beyond my ability to describe it. I wasn't able to comprehend it until I experienced it myself. And it's not a drugged experience, it is a combination of sober and clear head, but the most all-inclusive love amd acceptance for yourself and those near you. With no anger and no fear.
I have tried the odd opiate, and it's a nice, bit numbing feeling. But whenever I think of MDMA, I smile and get all fuzzy and dreamy inside. It's not a craving, it's a very found memory.
If you are determined to mix drugs, I really advice you to at least try the MDMA by its own first, and experiment next time. I also suggest that you don't redose, as this is much harder on your body and brain (particular neurotoxicity caused by elevated body temperature).
Two theories for neurotoxicity co-exist:
Elevated body temperature (keep it normal, dont sweat, is gived in my experience hardly any hangover)
Dopamine getting where serotonin aught to be (if that is true it's pretty bad news for me, I take stimulants for adhd)
To mitigate the negative effects on your heart by MDMA, I suggest you rather do a lower dose. Giving that your doctors have cleared you for adderall, I don't think your heart condition is any argument to take heroin. Particularly if you don't mix the two substances after your own tolerance (and you don't know your reaction to mdma), you have a higher risk of dampening/ruining your roll, or even worse.
