Serotonin syndrome will only arise when you start combining serotonin releasers, serotonin reuptake inhibitors or MAOIs that prevent serotonin from breaking down. I think the protoypical case of SS was phenelzine (MAOI) and meperidine (serotonin reuptake inhibitor) (Libby Zion).
DXM (serotonin reuptake inhibitor) and MDMA (serotonin releaser and reuptake inhibitor) is another combination that is known to cause SS.
Neither methadone nor MxE are serotonin releasers, reuptake inhibitors, or MAOI's. Serotonin syndrome is therefore not a concern unless you have very, very strange body chemistry.
Serotonin syndrome shouldn't be confused with bad trips, overdoses, adverse drug reactions, bad choices of set and setting, normal anxiogenesis from drugs that have that issue (stimulants, CB1 superagonists) etc.