If you don't live in the desert (like Arizona or Israel or something) then you don't have to worry about too much sun, at least not with older cacti or cuttings. I don't know about seedlings, never had those.
I have my cacti on a balcony that gets 8-9 hours of full sun in summer, in a German wine-region (that means it's a relatively mild climate for Germany). These conditions are almost perfect for the cacti, they grow like hell in summer.
For example last year, one 3 year old plant that had been cut 10cm from the ground grew one sprout that was 40cm and another that was 10cm in height - in only 7 months from April to November! And these sprouts are really thick bastards with 6cm in diameter.
I tried different kinds of soil, now I have special cactus soil from a cactus webshop. However the cacti seemed to like all the other soil I tried before almost just as well. Only one experiment with too much hydroponic stones didn't go as well, the cactus seemed to grow thinner in that. Normal potting soil with some sand and some seramis should be fine.
Fertilizer, as said before, should be low on nitrogen. One that I had from the supermarket was also lower on phosphorus and higher on potassium (3-5-7) and everything was basically fine with that. I switched to a fertilizer from the cactus webshop last summer which has an NPK of 6-12-6 (higher on the phosporus) and this really seemed to induce branching.
I mean a cutting didn't just grow one branch or maybe two like before, now it grew three or even four. Before that I had been really happy to see one cactus grow another branch, and suddenly they're all doing it wildly.
As long as we talk about fertilizer I might add that rainwater is always the best to water any plants. Especially if your tap water is chlorinated.
Hard water (=high calcium levels) obviously doesn't hurt them much, but as my cacti are outside they get their good share of rainwater (last year I had to bring them in once in a while because it wouldn't stop raining for days :evil: )
In winter, unfortunately, you have to stop the growing totally if you're in Europe or similar climate zones. Get the cacti inside, don't give them any water, and keep them cool. I kept mine in my bedroom last winter where it was between 12°C and 20°C (sorry, but I like to sleep at no less than 18-20°C...) and it was okay. Keeping it down to 10-15°C might be even better.
The year before I had them in my living room where it was always between 16°C and 24°C and that stimulated them to keep growing, even though they didn't get any water!
Unfortunately when they grow with not enough sunlight, they become thin. It not only looks bad but they are also prone to break next summer!
Wow, now that became a really long post!
However that should be pretty much all of it. San Pedro cacti are probably the most easy to grow plants that ever existed. Just look at the plants closely every day, watch for growth, color, how does the soil look, etc. Do this every day, so you'll notice small changes a lot better than if you just look at them from the distance (that is the secret of most plant-growing success, I think)
