As much as I like mescaline it would be redundant if you already have MDMA and LSD in the mix. Mushrooms, LSD and mescaline sort of fulfill the same role as a classic psychedelic and I don't see the point complicating a trip, pure is better. Some people might say the opposite, that they complement each other, I am not convinced.
Even if it were true rather than complementing drugs I like synergizing drugs, like a psychedelic + a dissociative + an empathogen + cannabis. DMT is a different story because it can overpower other drugs while taking something into the experience, for instance with 2C-B and DMT, I had visions of entities like I had before with DMT but now they were wearing neon clothes (with which they were very happy), not kidding. :D
Snorting 2C-B - normally very painful but less so while high on MDMA - can boost the trip in a different way than the oral use of a complete and full-on classic psychedelic.
Nitrous is also a category apart.
Another thing I could see adding to the general mix I mentioned is something like GHB - while it is classified a depressant it can make things even more fluffly and cloudy. Beyond a certain point with combinations it not only gets ridiculous but the chances of something going wrong also increase, mind you. Less is more at that point, to illustrate I would rather have a larger dose of 3 aforementioned drugs than the combination of 5, especially if those drugs are complete in and of itself rather than being a good temporary boost like nitrous.