It all depends on what you like and how much space you have. Cactus potting soil mixes are the best. It is also possible to just leave them alone. Let the water reach the roots and give the plant a drink. Healthy air plants can produce multiple offsets at once, so there's no reason to be concerned about your Tillandsia collection not growing. Once you've collected your cuttings, you'll need to leave them in the open air for a few days before planting. I keep my Aeonium Arboreum 'Zwartkop’ outside all year. They thrive in the winter, when it is the wettest season.