I have read that 5-HTP may increase the risk of serotonin syndrome when combined with SSRIs but this is not conclusively proven as not enough high-quality research has been done into the antidepressant effects of 5-HTP.
Data in this table indicates that taking 5-HTP and St John's Wort together will also increase the risk of serotonin syndrome: http://en.wikipedia.org/wiki/St_John...c_interactions
Personally I would avoid taking both these substances together. Speak to a doctor or a pharmacist.