I think I have mixed 3-MeO-PCP with almost every drug (MDMA, 4-FA, mephedrone, alcohol, heroin, weed, amphetamine, cocaine, tripts, phens...) without any strange reactions or negative side effects. Of course, the dose of 3-Meo-PCP I usually take is very low (1-2 mg snorted), so this might not hold for higher doses.
I just feel it polishes and smooths the crazy spikes of the other drug. Makes the experience more controlled, less anxious, more euphoric, more "intelligent" and less inebriating. It might be merely due to the positive psychological effects of 3-MeO-PCP, that make me feel more confident and comfortable, or it might be a true neurochemical synergy. I can't tell.
I also find it very soothing during the comedown of stimulants / empathogens. It just adds a kick of optimism and makes the end much nicer.
I also wonder, being this drug a NMDA antagonist, whether it has any effect in reducing or controlling the tolerance of other substances. I personally don't feel like my tolerance to other substances has decreased, but neither can I say it has increased.