That novel drugs are unpredictable in the first place should be a premise, but why not say it another time...
Combining stimulants is quite straining for the heart and veins, in many cases extra hard on the kidneys and liver and as I understand it also your brain...
Some accounts describe the wobbly state and peculiar side-effects 6-APB seems to have that MDA, MDMA and amphetamine (as a couple classic examples) don't seem produce. We still don't know enough about the stuff to make any certain statements but it's an observation that I got from only reading about different people who tasted it.
As far as combinations go I would recommend against anything with significant impact or debilitating effect.
On the plus side methylone doesn't make you that wacky. For an empathogen it essentially seems to be resembling the more typical stimulants (although there are those people who get a mainly stimulating effect from it anyway) - something like a good dose of MDMA can make you trip more out of your mind feeling above the ground, with a stimulant you tend to keep your feet on the ground so to speak. 2C-T-7 or AMT would be examples of things that would be very irresponsible to combine with 6-APB in my opinion and expectation.
On the other hand considering methylone is pretty stimulating there could be adverse cumulative effects...
Psychedelics and methylone can produce anxiety and panic attack like reactions when the methylone wears off.