Yes you can mix pregab and tramadol, just don't go crazy on the doses ie. Don't down 1200 mg pregab and then take like 600-900 mg of tramadol.
Keep it anywhere from 75mg - 150 mg - 300 mgs of pregab as a potentiation aid and try to lessen the usual dose of tramadol you would be taking. Remember, you can always add more if u want/need. Don't mix benzos with those two, unless you know what you are doing. Taking three CNS depressants at the same time can cause serious health complications like slowing of your breathing etc. (google it or search other threads). IMO
if you primary concern here is to potentiate tramadol use either pregabs or a benzo. Never mix those two.
To my knowledge there isnt any risk of Serotonin Syndrome mixing Pregablin and Tramadol. It's more like both can lower the seizure threshold. So if you fuck up with the dose or are prone to it you might have a seizure. Benzos are a good insurance against seizures(Pregabs also in the right dose). The problem is don't make it a habit. If u end up addicted to two/three substances at once it will be a really bad situation for you later when you want to quit, Next time write how much were/are you taking of
a drug, stuff like your tolerance level etc , It will help