Well, for alot of people here I'm going to be repeating myself, but here goes....
Your plants are hungry. They look pale green and if I saw that right, some of the lower leaves are yellowing. These are two classic indications that plants need food. The soil you've picked is fine, no problems there, although I wonder why you've mixed the two as one or the other will do just fine, but I'm not criticizing, just making an observation.
For a situation like this you can try almost any fert you want as long as it's a liquid. I say that because an organic powder fert will take time to break down and your plants need help quicker. The superthrive is nice but and even though I've used it, I have no proof that it really works. It is vitamins and growth hormones according to the label, and since hormones are very strong, you don't want to give more than one drop per gallon of water typically.
As for the fert it's almost impossible for this to be an over-fert condition because you say you haven't added any ferts. So, with that in mind and knowing that there's probably a fert added to the soils at the factory but it isn't much, then adding ferts is a good thing here. I usually recommend DynaGrow 7-9-5 for hungry plants. This causes some people to wonder if I'm a DG salesman, but ha ha, I'm not (but I play one on television).
The instructions for its use are on the back, it's just 1/4 to 1/2 teaspoon per gallon of water for a soil grow. Hydroponics is a full teaspoon per gallon. The reason why I suggest this fert is because it's a liquid and fast-acting and it has pretty much all the micro nutrients that plants need. For your plants I'd probably go ahead and use 1/2 tsp in a gallon, especially for the ones in bigger containers. There are many fert types to choose from but DG isn't super expensive and will give alot of bang for the buck. You can just try a fert that is sold at your local hardware store, but they don't have all the nutrients plants require. Funny thing is the manufacturers never say this. They expect the grower to buy the product and learn the hard way that it wasn't right to use.
You can try General Hydroponics fert, which will probably work find in soil or Supernatural Terra, which is in crystal form. Some people are using Advanced Nutrients. There are so many it hurts to think about it. Guess what an up and coming cottage industry this is! Supernatural has alot of fert components to it, but I've used Dynagrow and know it works. Just don't overdo it. There are even powder ferts like Foxfarm's Fruit and Flower. These come in a box but you'd have to transplant your plants into bigger container after having mixed the powder fert in with more potting soil. These ferts are good to use but your plants need help now. So find a hydro store near you and try to get DG or something similar, making sure that it has all the boron, sulphur, magnesium, manganese, iron, etc., if you pick a differnt fert. That's the best I can give you right now. Maybe someone else can pitch in and say what the think. Good luck.