I have been doing organic soil mixes for my Pedros, I have many torches from seed and some around 1 year and they are growing great(ordered offline at around 2-3 inches and all are now reaching a foot) ! I mix about 50/50 organic potting mix and perlite to start. Then I add around 1 cup of earthworm castings, and 1 cup organic compost. Make sure to sift compost and soil for sticks and debris as it robs nitrogen.
Then I mix
4 tbsp. rock phosphate
4 tbsp. green sand
2 tbsp. bat guano
1 big handful of coir
BTW this is for around 20 gallons worth of soil, Give or take for bigger or larger pots, or just make a whole bunch of mixed soil at once for multiple pots.
Coir (Shredded coconut husks) helps seedlings as well as any thing develop fine root systems
Make sure to mix all your ingredients well, avoid and clumps of amenities in soil. After you pot your cacti you can give them a nice layer of sifted compost ,give see I add a tiny bit of eartworm castinngs as well but that's optional I suppose. Put a nice layers of pebbles or anything to that effect to keep perlite and other nutrients from floting around during watering, and let your little guys thrive
They can be watered up to twice a week in summer, I add Schultz cactus juice to my water. Either 7 drops per quart per cacti or 2 dropper fulls to water multiple catus with bigger watering jug. I use a half gallon jug that I can water 4-6 cactus with with the 2 droper full water. Winter do not water unless they look obviously in danger.
Hope this helps, mine grow as fast as can imagine them growing, and least a one to two inches a month, which is great for me. The more love you give your cacti the more love they will give you in return
love and light