I haven't had experience with those exact drugs, but have been on moderately high doses of zoloft and high doses of imitrex simultaneously. I didn't find it to be a problem at all.
Whether or not serotonin syndrome strikes has a lot to do with: specific med, dose and individual physiology. If the Effexor is low dose, and your neuro has given the green light, then I would say give it a shot. If you're feeling uncomfortable you can always try taking a half dose of the maxalt.