Toss the seed starting mix and moisture mix into the garden where it won't do any harm.

If cuttings, you should be using a very small container and a coarse potting medium.

mg cactus mix will be totally fine. keep it moist while it's rooting. but you can easily root in water too.

Peaty potting media like MG moisture control are far from ideal, but spider plants can tolerate them just fine. Use what you have and don't worry about it.

Soils that hold the least perched water (water in that soggy layer of soil at the bottom of the pot or 'perched' above drainage layers) offer the best opportunity for plants to realize/grow to their genetic potential. You can 'make' just about anything work if you have a mind to and are willing to work at it, but soils comprised of larger particles offer a home where roots can be happy ALL the time - not just when the soil finally dries down enough to allow enough air to return to the soil that the air:moisture relationship doesn't over favor moisture to the degree the relationship ensures limitations.

Spider plants don't like wet feet; they don't like a high concentration of solubles in the soil; and they are known to be sensitive to compounds containing fluorine.

Delineation between plants that grow and look good and plants that merely survive is often determined by whether they're grown under conditions they tolerate vs conditions they appreciate. If you provide conditions that put the plant growing in its sweet spot (soil make-up, light, temperature, moisture/fertility levels ....) they are certain to show off their appreciation. The most important decision you'll make in establishing your plantings is probably soil choice ..... and it's actually soil choice that determines what size container is appropriate. With the right soil, you can plant the smallest slip of a plant in a 55 gallon drum if that suits you.

Al

If you want to give them a good soil, adding some large particles to a peaty mix won't help. We tend to see a soil as a collection of particles, but water and air see it as a collection of spaces. If you have peaty mush in with larger particles, the spaces between the large particles will be full of peat pudding, which isn't much different from having the whole medium be peat pudding.

For more than you probably want to know, see Al's full write-up.
http://forums2.gardenweb.com/forums/load/contain/msg0616523214714.html

Here is a link that might be useful: Container Soils - Water Movement & Retention XVII