Cannabis is already a psychedelic substance by itself. There is a nice overlap between the effects of cannabis and of classic hallucinogens like LSD and shrooms. So it's no surprise that they tend to be synergistic in a way. How this synergy works out though, will differ for each psychedelic and it will also depend on method of administration, dose and so on.
The main point is something that may surprise some people: it's the cannabis that you have to be most carefull with.

I personally have had experiences with cannabis combined with LSD, shrooms and ayahuasca. I suppose that mescaline, dpt, etc, would not realy behave very differently with cannabis.
The ayahuasca i will not discuss because harmala's also play a significant role here.

I find LSD to be a better match with cannabis, than shrooms. This is because LSD is a stimulant as well. The main difference between LSD/cannabis and shrooms/cannabis, is that with shrooms, you realy can get a somewhat foggy or drowsy kind of state of mind. Though the combination of cannabis and shrooms is extremely powerfull, and it will make you hallucinate like never before, it sometimes feels like you're not realy awake and it's all just a dream. Once i even fell asleep when i had taken this combo. With LSD you don't have that. With LSD your mind will mostly remain it's 'presence'.

There's a difference between oral cannabis and smoked or vaporised cannabis. Smoking cannabis is realy nice on psychedelic's, because you realy experience some sort of rush when it hits your brain. But the effects of oral cannabis are far more powerfull and profound.

With shrooms, it is best way to take oral cannabis, is to dissolve cannabinoïds in vegetable oil. This is because large quantities of food will slow the absorbtion of psilocybin and psilocin, into the bloodstream. A small amount of oil can contain large amounts of cannabis. But i would advice to be very moderate with the cannabis. It's the cannabis that can be unpredicable and overpowering here. When too much cannabis is taken, you can become extremely drowsy and sleepy, and you can even fall asleep. You can see the most wonderfull things happen right in front of you, but hardly realise it. But visually, the combination of shrooms and cannabis seems to work better than LSD with cannabis. If you ever want to see exploding fractals all around you, then shrooms and cannabis will do it for you.
Cannabis also alleviates side effects like vasoconstriction, nausea, shivers or stomach cramps. Actually, there's nothing that works so well against these side-effects, as cannabis. Luckily, even small amounts of cannabis are effective there, so you don't need to take a lot.

While with shrooms and cannabis, the effects of both are realy hugely amplified, with LSD it's more that the two substances melt together and alter the character of the experience.
The LSD experience is altered and it becomes warmer, friendlier and less speedy. A moderate amount of cannabis will simply make you more relaxed, and it's easier to open yourself to the experience. With both shrooms and LSD, cannabis is somewhat of a lubricant, making visions and thoughts flow more naturally and fluïdly. But with LSD, this aquired smootheness is the most pleasureble, if you ask me.

these days, when i do psychedelic's, i prefer not to take cannabis, because i like to stay present. But oral cannabis with LSD or shrooms is realy worth trying. I'm glad that i've experienced it.

When it comes to oral cannabis: you'll need half of what you'd think you need. Anything less than half a gram works realy well, and will just smoothen everything. Anything over a gram will be rather like adding dynamite. And anything in between a gram and half a gram, will be something in between adding lubricant or adding dynamite.