Yes! Corkbark is what I have with mine, and it doesn't mold. You can generally find it at petstores i believe, but if you get them from ken I'm sure he may have some pieces to give ya, just ask.
I keep mine on my snake rack so they are close to a nice ambient temperature of 78-80 or so.
I was told to dribble water down the side when they are small, moistening the substrate on one side, and once it dries up almost all the way, do it on the other side. I like seeing visible droplets on the side of the container, just a few, so I know there are droplets of water to drink. Don't mist the spiderling itself though.
What temperature do you keep your home at?
I know what you mean about the p irminias. .. I'm getting one at the end of the month and I'm so excited. I have two beginner species right now and I'm getting 4 more a bit more aggressive species, since I feel more comfortable now.