What ive found personally is that after taking mdma i can vape nearly twice the amount of weed and still not be quite as high.
Im not quite sure of why this is, but im just saying its like someone increases my tolerance to weed overnight or something.
I personally like to use Cannabis approx 6 hours after taking mdma, it gives me this dreamy high when i shut my eyes. If you are a very experienced cannabis and mdma user, then edibles might be okay, but if your not there is a chance you will have an anxiety ridden high.
The THC in small amounts, might be able to help you produce more serotonin over a period of time, but smoking, vaping or eating cannabis after mdma, i wouldnt have thought it would have any significant impact on serotonin levels. Mdma is a potent serotonin releasing agent.
Best of luck whatever you decide to do, but me personally, id say just have a smoke, chill maybe lay in bed with some music playing. I get these awesome scenic CEV's (Closed Eye Visuals) of beaches, mountains, meadows etc.