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I think you raise a valid concern. Inducement or exacerbation of schizophrenia symptoms is probably the most troubling and widely reported problem with these drugs. The famous case of Syd Barrett comes to mind. There are many such anecdotes, and I myself saw something similar first hand (the individual in question eventually got better, but it took a few months). However, considering the potential upside, to me this makes it even more important to do extensive research on all of it.

Oh, and a quick note-- the only 'psychedelic' drug mentioned in the study you cited is cannabis, which from everything I've read doesn't even begin to compare to the more powerful psychoactives. Funny enough, it is the one we have the strongest link to psychosis for, but this could only be because it's been so much more widely used and studied. It seems obvious that more study is needed, insofar as it can be done ethically.