Id give it a few days personally, i had a really bad experience one time taking Amphetamine and Tramadol close together. I had been up all day and night tweaked out, drank myself to sleep for about 6hrs. Then i woke up and popped 400mgs of Tramadol cuz i was hungover and had definite Serotonin Syndrome symptoms it was awful. To the extent i would be crazy careful dosing Tramadol after making sure it was nearly outta my system completely before i fucked around with any drugs that played off seratonin.
Ohh and Ketamine will do nothing to prevent seizures or SS from developing. The biggest risk of Seratonin Syndrome is when your body overheats because your organs start to fail and it can cause permanent damage and obviously lead to death for some.