I agree wholeheartedly. Get those plants repotted.
Seed starter is just that, a seed starter. The seedlings need transplanting before the nutrients in the starter cotyledon leaves are consumed since seed starter lacks nutrients. Some seed starter has mild, short acting nutrients added, but that only lasts a week or so.
To obtain the advantages of seed starter and avoid the need for immediate transplant, I use soil on the bottom 2/3 of starter cup, with seed starter on top. The seed starter makes it hard to overwater and compact, but once the roots develop they reach the soil where they can find nourishment until time for transplant.
You really need to get yourself a pH meter before you start feeding your plants. The fox farms nutrients tend to be slightly acidic so you need to adjust pH before feeding. pH meter not as necessary when watering, but critical when you start feeding.
To use the Fox Farms nutrients you need to download yourself a feeding chart.
Based on week you are in, you mix your nutrients per the chart. Start with a gallon of water and add the nutrients in the recommended ratio. For example, say you were on week 3, you would mix 6 tsp on Big Bloom and 3 tsp of Grow Big. Before feeding you need to adjust the pH to the suggested range of 6.3-6.8.
Since you will be transplanting to new soil you won’t need to feed at this strength or at all for a while so you can dilute the mixture by adding another gallon or two of water. Left over nutrients can be kept for a short time or fed to other house or garden plants.
After every 3-4 weeks of feeding Fox Farms nutrients you’ll need to do a flush to avoid buildup of salts. When you get to that point tag me and I’ll explain your various flushing options.
There is a bunch of info on the chart that I didn’t explain so if you have questions ask.