No, there wouldn't be much nutrition, if any, in that mix, but it's also hard to overwater, too, right?

As a non-seed-grower, I'd guess a weak solution is OK.

Of the various kinds of cacti seedlings I have, most need a bit of water every two days and that is with the dome on the seedling tray. Strangely, the Escobaria organensis seedlings take several days to dry out but they haven't rotted, either. The Echinocereus engelmannii variegatus seem to grow quickly, the quickest, regardless of the poor nutrients. Yes, I made the mix with keeping it in a humid environment in mind. In past misadventures of starting cactus seedlings I've found that if their soil mix dries out in dry atmosphere then they quickly shrivel away to nothing. I was hoping to keep them in a more humid environment, which seems more forgiving if the watering is slightly neglected, until they have a bit more size. So far they are looking good. Only a few seedlings have died. There were a couple that looked odd that I thought of grafting to Pereskiopsis rootstock but waited too long and they shriveled away and a couple others that looked OK but didn't grow roots for mysterious reasons and shriveled away. I wasn't sure if I should not interfere and just watch them grow, even if slow, but was leaning toward trying a weak fertilizer.

I bought some Schultz 2-7-7 for cactus and followed the box's guide for mixing a strength meant for feeding every time I water.

My cactus seedlings are in a mix of fine pumice with a little bit of coir. I keep them enclosed in clear plastic boxes until they're about 8 months old. it's really damp and humid in the boxes. If they start to look a bit dry, I spray them with some water.

After I start taking the lids off, and they start drying out a bit, then I start feeding them with a dilute dose of dyna-gro, probably about every third watering. After they're uncovered, I water them every few days.

I mostly have Astrophytum, also some Ariocarpus, Mammillaria, Gymnocalycium, Lophophora, and two lonely Copiapoa seedlings. Oh, and they're all under lights.