Bupropion + SSRI is a fairly common combination for treating depression! Back in the day I think people used to start on an SSRI alone and then would add bupropion if they needed an extra kick, but it may be more common now to start with the bupropion (not sure).

And re. serotonin syndrome, I can anecdotally say that I accidentally gave myself mild SS by mixing imipramine, ondansetron, and NyQuil. I suspect that also being on bupropion at the time is what put me over the edge, but it's hard to say; even without the bupropion it may have been a nasty combo. Live and learn!