SSRIs are not a problem in combination with MDMA. They will not lead to serotonin syndrome, they merely blunt the empathogenic effects of the roll. This is a combination that many people, including myself, have taken safely over the decades.
When combining SSRIs/SNRIs with MDMA, you will still feel the stimulation and the trippy effects attributable to the drug. However, in the case of MDAI, it is - as I understand it - solely a serotonin releaser, so combining SSRIs/SNRIs with it will likely produce little if any effect. Still, it won't be a dangerous combination since it's not an MAOI.
Just to explain this a little further in case you are confused, the pharmacological action of the SSRI prevents the serotonin efflux normally provided by MDMA or similar at the very same receptor, thus negating the danger of extra serotonin building up in the presence of an SSRI. Think of it as a uni-directional revolving door, whereby MDMA reverses the direction of that door and pumps serotonin back out of the neuron in to the synaptic gap. SSRIs merely place a chock in the door, preventing or slowing the flow of serotonin in either direction (the normal "direction" being into the neuron). The MDMA is trying to spin the door in the opposite direction to "normal" flow, but there's a chock in the door preventing it from doing that very efficiently.
If, theoretically, you took a large enough dose to overcome this particular action of the SSRI, you still wouldn't be in danger because the MDMA would then be overriding the effect of the SSRI that you were concerned about, thus still negating the the worry of dangerous serotonin build-up.
The only thing you have to watch out for is serotonin releasers which are also MAOIs, such as aMT. These are indeed contraindicated with SSRIs and similar drugs. Also, 5-HTP is contraindicated with strong SSRIs because it overrides the rate-limited enzyme conversion of tryptophan into serotonin, producing a rapid rise in serotonin not exacted by the mechanism of efflux as seen with MDMA. I have yet to see any serotonin syndrome reports of 5-HTP with SSRIs, but the possibility exists and they are clinically contraindicated.