You're correct, Effexor does not have a high affinity for serotonin, especially at that dose. Chances are since you just started Tuesday, I would guess that you would still roll, but you may still have reduced effect. Even though MDMA is targeted towards serotonin it causes releases and effects on Dopa and Nor as well, so I still would predict a different experience.
That all said and done, your body is currently trying to adjust to the newly added Effexor. I'm unsure what the consequences of rolling at the beginning of starting it will do.
I know someone who was on Effexor, at a higher dose, for much longer, whom took a good 140mg of high quality molly and did not really roll. They claimed they felt relaxed and peaceful, but I wonder how much was placebo; their eyes were not even dilated throughout the whole experience. They did, however, have an imbalance of mood and emotional stability the week following.
I'm guessing you're probably going to do it one way or another, it may be the last time you can while using Effexor, and for a period afterwards. I would caution against it though.
IMHO anyone who is prescribed SSRI/SNRI/SDRI medication should probably steer away from these types of drugs, chances are it will just disrupt the balance even more. I'm guessing the onset of your Effexor's beneficial effects may be altered and stepped back by doing so.