I usually keep them in high humidity until they're repotted, or when they have grown about an year. As for fertilizer, I haven't used it on seedlings before.

Thanks for the info. About how big do they get after a year, marble size?

Haven't tried Pseudolithos yet, but I know that this method makes plump Ariocarpus seedlings....You might have to remove yours a bit earlier.... Probably not too big after an year though, make sure they don't rot, that's the main factor.

The possibility of rot is why I'm concerned about their being in such a humid environment. All I've read stresses dry conditions. I guess I'm going to have to keep a really close eye on these. I have a mature specimen that I got around the same time as the seeds. It's in a similar potting mix but obviously uncovered and no bottom heat. It's under T5 lights. The container is quite small, a saki cup, and I water it every couple weeks. So far it looks good but is probably the same size as when I got it. It does have a small baby that might have gotten a little bit bigger. How big/old do these have to be before they bloom and is it difficult to get them to bloom? I haven't found much info about it. I just saw a pic that someone had posted and thought it was such a cool looking plant.