I agree with Rasta, with Amanitas I'll try to take at least 35 g, even though that's above and beyond the "strong" does as classified by Erowid. Fly Agaric's are one of the most variable drugs, so 12.5 might be enough for you, it's very individual. On to the San Pedro, T. Pachanoi (sp?) has been a let down for me, and at only 12.5 g it'll be a waste most likely. If you can't find Peyote, which is far and away my preferred mescal source, then try to get your hands on T. Peruvians (sp?), as I believe it to be the more potent legal cacti.

On combining, I wouldn't recommend it. I'd only combine hallucinagens if they have the same active ingredient, such as Hawaiin Babies and Morning Glory, otherwise it really muddles up the whole trip. This is of course assuming that you aren't doing it just to "get high," and if that's the case then hallucinagenics in general really shouldn't be your poison.