Fertilizer isnt going to hurt them - soilless is recommended because it is generally sterile, so it wont introduce possible fungal virus.

A lot of mixes are peat or coir based, they would be fine to use.

I dont worry about it to much. I use the same mix that I pot my seedling up into, which is a mix of cheap potting mix, pinebark fines, peatmoss & pearlite.

Pro Mix BX and Fafards mixes are the commercial mixes I have seen recommended most often on these forums. If your local places carry them, grab some.

I would avoid mixes that have water retention chemicals added. Dahlias rot if they have too much water. The little bit of fertilizer that they add to potting mix is probably OK. I always say growing dahlias in pots is not for the weak of heart. Lots of problems that are not encountered when they are planted in the ground. A good compromise is a smaller pot(1 -2 gallons) buried mostly in the ground. The soil temps are cooler and there is less need for watering all the time. They do as well or better than dahlias planted in the ground.