You will not be at risk of serotonin syndrome. There are even indications that an SSRI (like prozac) lower or even eliminate the chance of serotonin syndrome compared to MDMA on it's own. This is because of it's method of action. However if sufficient levels of prozac have built up this same method of action will indeed mute or completely kill the roll. Furthermore not much is known about the combination of MDMA and an SSRI, apart from sparse research, so even though serotonin syndrome is not a risk, there are a lot of unknown factors that could be. There have been no, or very few reports of adverse reactions but you can never be certain with two such powerful substances combined.
As for the post-roll protection an SSRI offers, IIRC this is a theory but it hasn't been sufficiently proven. But if you keep your doses normal it isn't needed as normal doses do not cause neurotoxic damage. An SSRI after MDMA is to combat neurotoxicity, not to help with the comedown. A comedown is not neurotoxic damage but a drop in levels of serotonin and other hormones in the synaptic cleft. It is my understanding this cannot be counteracted by an SSRI, only neurotoxic damage (possibly)
Not really important in this case but if you ever switch from an SSRI to a MAOI anti-depressant, DO NOT take MDMA, this is a potentially deadly combination (it has been fatal in numerous cases) and will at least guarantee you a trip to the ER.
Now that I mention it, don't combine an SSRI and MDMA either, in case that wasn't clear from the rest of my reply
besides the potential adverse reaction it is not wise to do so if you are undergoing threatment, as Daiki pointed out already. MDMA fucks with your hormone system, this you don't need when it is already a bit out of balance... You will feel better during the roll, but the crash afterwards can send you way down. First take care of your issues before you contemplate taking drugs..