I can’t find a clear answer for this on the web. So for example, I take creatine HCL which is extremely acidic (mono creatine isn’t). Aside from the obvious that something very acidic can upset the stomach by itself, could an acidic substance specifically interact poorly with medications or vitamin mineral supplements or other supplements, such as Ritalin, lamictal, multivitamins, etc? Like is grapefruit juice said to be bad with meds because of its acidity, or is there something really specific about grapefruit juice that makes it bad with meds?
Thanks gang 
It’s not the acidity in grapefruit juice that interacts with some meds. Grapefruit juice inhibits CYP3A4, a liver enzyme which metabolizes a number of medications. So consuming it can cause elevated levels of the med, and hence problems.
The stomach is an extremely acid environment on its own. Adding extra acid in the form of food and drink won’t change what happens to the meds ingested.
I think if the medication is approved for oral use, it means it remains effective after being exposed to stomach acids. Which can be more acidic than any food you are likely to eat.
Supplements aren’t subjected to the same rigorous approval process, so who knows…
Grapefruit interacts with some medications, but not because it’s acidic.
One hopes that the OP is taking a creatine supplement approved by his/her physician for a medically indicated use (of which there are a few, though mostly people use it based on mixed data suggesting it might help athletic performance).
Acidic supplements and foods/juices aren’t going to change highly acidic stomach juices. They might worsen esophageal reflux symptoms (some acidic foods/juices are on a list of factors potentially having a negative impact on reflux).
A handy Google search will turn up potential creatine-drug interactions.