Unless the plants are already 1ft+ tall then you will definitely burn (and possibly kill) anything smaller in supersoil. If the supersoil in question has anything like Blood/Bone Meals, any type of Guano, or Alfalfa Meal then you can definitely burn plants using those amendments. I'd consider looking elsewhere for your business in the future if the company you purchased from said supersoil won't burn plants, they're setting people up for failure with that kind of advice!
The energy from the decomposition process can make things as hot as 150 degrees, and if you put a clone/seedling or even a 1ft tall plant in that soil and expose the roots to those kind of temps then you will for sure have issues. Deformed and stunted plants at best, and dead plants at the worst.
Assuming this is SubCool's Supersoil recipe, you'll want to do the following. First, you'll need a base mix that isn't so heavy on nutes. Mix it yourself with peat, perlite, and compost. If you can't build your own base soil then I recommend Fox Farm's Happy Frog, only bag I know of that comes in 2cuft instead of 1.5cuft. Happy Frog has good stuff in it and is my go-to when I'm buying bagged soil, which is quite rare these days. You want to put the supersoil at the bottom 1/3 of the pot. On top of that, 1/3 of supersoil mixed with base soil. Then, the final top 1/3 layer of soil will be purely base soil. This way your roots won't be burned because it won't be exposed to the supersoil pre-maturely. It will grow through the base soil into the base+supersoil mix, then finally into the supersoil in the bottom of the pot. Nothing will burn this way.
As for liquid nutes, save your money and don't even buy them. Unless you're using Fish Hydrolysate, Fish Emulsion, Liquid Seaweed, or Silica then you should stay away from bottles. The reason is because chemical based nutrients will kill the microbes in your super soil, effectively defeating the purpose of purchasing the supersoil in the first place. The idea behind supersoil is water only. You can top dress with Neem Seed Meal (6-1-2) in veg, then switch over to Crab Meal (2-3-1) or Guano (0-5-0) in flower and that should really be all you need. If you have heavy feeders, Fish Emulsion in veg and Fish Hydrolysate in flower.
Look into Clackamas Coot's soil recipe, which is a revised version of Cornell University's recipe. Equal parts peat, compost, and perlite. The only amendments are Crab, Neem, and Kelp Meals with some sort of mineral amendment, typically basalt, glacial rock dust, or azomite if you can't find the first two. Mix is ready to go the second it's mixed, and will not burn your plants. Been my go to for the last 4 years or so now. I top dress every month and use one of the bottled products mentioned above if I'm dealing with a heavy feeder.
