The reason you would use cloning gel is to get the plant cuttings to root quickly. You want to get the cuttings as close to the time of transplanting as possible, which means that you don't have to wait for them to grow roots before you can move them into a larger pot. If you are using soil, then you will want to start them in something like Miracle-Gro Root Booster (or equivalent) as it will give them the right amount of nutrients. Potting mixes will work but if you're starting from seed, it's best to avoid potting mixes as they contain too much sand, which could cause problems with the seedlings' root systems.