Supplements like meds I think are a crap shoot. We all metabolize and synthesize so differently. Trying it to see if it helps or not is all you can do. I wouldn't think it would down regulate as it is needed to synthesize serotonin. I am not sure it would cause more serotonin because your body would just use what is needed. I think most people get enough in diet.

You might try L Methylfolate as some people lack a gene nessacary to turn folate into serotonin. It is prescription only, Delpin. If you are taking meds you have to be careful with supplements.

Ask pdoc and look up the chemistry of the supplements.
Sam E
L Methylfolate
L Tryptophan
Melotonin
Vitamin D

I see you are not on an SSRI or SSNRI so I wouldn't worry about too much serotonin.