With MXE or K combined with something like LSD/shrooms the rule I use for candy flipping applies. Basically, there really isn't a wrong way. I prefer the LSD first and the MXE once I'm on the peak. But I've done it the other way around and many of my friends preferred the disso first because it takes away some of the come-up anxiety.
3-meo-pcp was a different beast all together. I never combined it with a psychedelic but I found it incredibly manic even with just one re-dose. I would be more careful with it than I would be with MXE/K. Sedative dissos can take the edge off other drugs where manic ones like 3-meo can really through you for a loop and increase anxiety a lot. I've seen multiple people get into a bad place with those. They keep re-dosing without realizing how gone they are. Since the disso doesn't sedate them they end up walking around like zombies with no idea where they're at.
One of my friends blacked out on one of the 3-meo's and attacked someone while he was in that state. It was totally out of character for him. The lights were on but someone else was home. He didn't remember anything from that night (or the few nights before probably) and it screwed him up pretty badly. He attempted to choke someone to death. If other people weren't there he would have probably killed the person. Even though everyone forgave him for it I don't think he ever got over it. I'm convinced it was part of the reason why he drank himself to death a couple of years later.