You still are going to need to feed.
Perferted soil is a pain in the ass IMHO. I like COMPOST, but avoid anything with added nutes like the plague, unless I added them myself in the form of bat guano, bone meal, etc.
And when you flush, where do you think the ferts are going to go? They run right out the bottom and you might as well be using non-fertilized soil.
Plus most of those mixes are too strong for seedlings. Start in something more gentle like happy frog of a generic seedling mix.