On interactions with psychedelics: I don't stop taking any of the above when I trip, and I have never noticed any interaction (positive or negative) from any smart drug or supplement. Adrafinil seems to make my head clearer whether I'm on psychs or not, but it is an especially noticeable effect on tryptamines -- I don't know if that's an "interaction" per se.
As far as long term health risks -- all this stuff is completely uncharted territory, and if we wait around for clinical studies we'll all be dead. Nobody is applying for research grants on the interactions between piracetam and 2C-E. Like another poster mentioned, I tend to go with my gut. I feel if one is not willing to take that kind of risk, it is foolish to take any of these substances, together or separately.
Oh, speaking of 2C's, 2C-D is a very definite nootropic for me at small (10-15mg) doses. I have even, once or twice, gotten that rare and amazing feeling of knowing exactly what to do or say in any given situation, as though all the gears are turning without impedance and one's gestalt "true self" can manifest clearly. However, 2C-D builds tolerance quickly, and I am uneasy about taking it on a daily or even weekly basis. It's a very strong drug and not to be underestimated.