This little garden thingy is very temporary. Looks great to do for short stuff like radish, watercress but plants that grow larger will not get enough light to make their own food with which to use for growth and reproductive grow. Squash would not be good for example. The leaves are not able to get enough light.
Perhaps if you were trying to grow stuff out on your patio? Covered patio? This would work. The only soil that should be used for any and all potted plants is plain sterilized potting medium.
Definitely get the cheap plain jane sterilized potting medium. Make sure you don't get fertilizer added or water holding gimmicks such a sponges and gels.
Note: Just went through your second link. This looks like great stuff to clone plants. Proper directions as far as I could see. And you are correct, this will NOT support the plant any farther than two leaf sets.
Once you see roots coming out of the cube, transplant that cube and plant into a 6" pot using plain plain sterilized potting medium. Medium should be 1 inch below rim for watering. Hole in bottom of pot. No rocks or gravel below soil. Raise the pot off surface using 1/4 inch pieces of tile. Feel the heft of the pot/soil/plant when watered fully. Do not water again until that pot/soil/plant when lifted again feels OBVIOUSLY lighter.
Use a tiny bit of fertilizer and I recommend for newbies (and myself) Osmocote 14-14-14 all purpose, extended release fertilizer. Use half of what the directions recommend.
Make sure you have ventilation meaning moving air all around your plant and plants. Water during the day, not at night. If you are indoors or a green house you have to use the proper lighting at the proper height, maintain the heat 55 - 70 F., keep humidity stable and use a fan that blows YOUR hair around for ventilation.
To replace the soil, simply use sterilized plain jane potting medium!