It's not really about the size of the container yet but not a bad suggestion since you will be changing soil anyways. You will eventually need a larger pot so now is as good a time as any. Right now though it is about the health and growth of your plants.
Fox Farm Ocean Forest soil is a great suggestion for soil however since it is mass produced it can be inconsistant. I use FFOF but I mix it with coco. I use a 60/40 mix of coco/ffof this mix is very plant friendly and the roots love it which means the plants love it as well.
If you do end up going with the coco ffof mix try finding the dry compacted block of fine coco fibre. To use it put it in a large tub and treat with cal/mag water ph'ed at 6.0 before mixing with the soil. It will take several gallons of water and each gallon will need at least a teaspoon of cal/mag. The coco will absorb all the water real quick.