Oxy is pretty commonly faked. Several EADDers been burned buying fakes. But is probably rare enough here to not be likely to come across it. Know your source as with all drugs. I've only had UK scripted oxy though so probably wouldn't be the best person to ask, Dan.
As for serotonergics laying fine, it kinda depends on the serotenergics in question. As far as I recall, the APBs have some potentially iffy actions combined with other serotonin releasers - of which Pirate is definitely one. I'd not mix it with MDMA, for example, which is not a million miles away from meph in some regards. Most reports of combining 6-APB with other serotonergics have been pretty iffy, from memory. Am pretty sure meph would come under the same broad heading of Drugs Not to Mix Wiv APBs. But am no chemistry bod so sure somebody can correct me if mistaken.