Taking 5-HTP with MDMA has never been reported to cause a single incident of Serotonin Syndrome, and I can't imagine why it would. 5-HTP causes a small boost in serotonin levels in the brain (much of it is converted to serotonin too early in the stomach, and serotonin can't cross the BBB, but some of the 5-HTP does and converts on the other side), not nearly enough to cause SS issues, and it doesn't have any effect on the metabolization or oxidation of serotonin or MDMA. Maybe if you took many, many doses of it, but plenty of people take it with or during their rolls and not one of them has been known to have Serotonin Syndrome result from that combination. How effective 5-HTP is at boosting a roll seems to vary a lot from person to person. Pre-loading in the days leading up seems to have a positive effect for me, but it's hard to say really.
I've never tried dosing a significant amount of 5-HTP while rolling because of reports that it can mute the effects by triggering receptor downregulation somehow, although on a recent roll I emptied a 5-htp cap since I didn't have any empty gelcaps handy and wanted to cap up my molly dose. At least a few milligrams of 5-htp was stuck to the inside of the cap, but it didn't have any muting effect at that tiny dose at least - I rolled face that dose
I suspect the primary real use for 5-htp, if any, is post-loading to speed up the rate of replenishing serotonin levels, but it's not really very effective since there's an enzyme in the gut that converts much of it to serotonin before it can cross the BBB.
Vitamin C doesn't potentiate it (although maybe indirectly you'll feel a bit better/healthier after a nice big vitamin C dose, which certainly wouldn't hurt your roll), but it does probably reduce the neurological damage MDMA is theorised to cause based on oxidized dopamine damaging 5HT receptors. Vit-C is probably the best candidate for a healthy, safe, cheap and easy to acquire anti-oxidant that, in at least one form your body produces out of it, crosses the BBB and has been shown to block nearly all neurological damage caused by MDMA in rats. I always take vitamin c with my rolls when I can - no reason not to, your body can safely process a near infinite dose of the stuff and it's good for you anyway. I like to take those Emergen-C packets - they taste decent enough, just a mild citrusy fizzy taste in some water, and they have not only C but various B vitamins, other anti-oxidants and electrolytes - all things it doesn't hurt to load up on before and during a good roll!
