According to my country's health advice site you should not combine Citalopram, Mirtazapin etc and l-tryptophan. Says nothing about how likely serotonin-syndrome is with the combination, only that you shall not combine those becuase of the risk.
I'd advice you to skip the L-tryptophan as you wanna try the SSRI and see if it works without any other medication in the mix. To be honest, stay of drugs untill you have evaluated weather or not the SRRI works for you or not, otherwise its kind of pointless.
Wish you the best of luck!