So I know that fungi and cacti generally shouldn't co-exist, but I'm curious how bad these fungi are for my San Pedro?

His soil got a little too damp, but I've put him outside to dry up a bit. I feel bad as that will obviously upset the fungi, but unfortunately this isn't their home. My main concern is root rot, though the Cacti seems to be doing fine.

The little yellow ones are Leucocoprinus birnbaumii, a very common species in indoor flower pots. They've grown in some of my San Pedro pots on occasion. I'm not sure about the grey ones but in all likelihood they're growing on the compost and nothing to worry about.

San Pedros are very tolerant of moisture and, indeed, will grow like the clappers when given plenty of water and sunlight, especially when fed every 4 weeks with half-strength tomato feed. Thus, drying them out here is not strictly necessary - it would be interesting to find out what the grey mushrooms are - I once had Panaeolus cinctulus (syn. P. subbalteatus) sprout out of one of my flower pots...

Root rot is as likely to be bacterial as anything (that's what keeps getting my Stetsonia coryne specimens - cold plus damp really isn't their thing). Fortunately, San Pedro seems to tolerate wetter soil and lower temperatures than most other cacti.

So, it seems you have nothing to worry about, rather you can be generous with the watering and find out just how fast your cactus is capable of growing!