seeking experienced growers knowledge

ok so I got these seeds from a amazing bag of weed i got and germinated them. After they opened I put them in ffof soil not knowing it was to strong and six out of those 8 died of nute burn. of the two that survived they are in a cup at the moment. They didn't suffer any nute burn but they are getting to big for the cup ( they are 3 weeks old). Heres where you knowledgeable growers come in I'm worried if I transplant my plants into a bigger pot and give them ffof that they will end up getting nute burn bc the pot will be much bigger than the cup thus resulting in more soil thats super strong. i also bought some ff light warrior soil after the 6 plants died however im worried if i put them in that soil it'll throw off their ph or something bc im mixing the two. I also gave them nutes for the first time today grow big (6-4-4) the feeding schedule asked for 1 teaspoon per gallon but i gave it 1/4 tea spoon. I'm not sure if that'll play a roll in transferring them. Thanks for any beneficial input.

Also, should I transfer my plant to a bigger pot as soon as i notice its roots poking out to edge of the soil? I heard not moving it when the roots are coming out the soil will limit its growth

you can transplant into either the light warrior or the regular FFOF.

if you want to guard against nute burn, cut the medium with some perlite, about one part perlite per two parts soil. will also help drainage with FFOF, which tends to get very clay-like.

you don't need to transfer as soon as you see roots at the bottom, but you shouldn't wait too long.

Transplant it asap and yes ffof is way too early. I start off with sunshine advanced. Make sure the holes u make arent too deep cause they will die trying to reach the top. The nutes are strong ùse very little to a gallon but dont over water stop as soon as there is water coming out ìf the bottom. Make sure your not using white cups or anything the light or sun can penatrate the roots dont like it. I use organic and its very mild stuff i start feeding at about two weeks old. Make sure your nutes are for vegging and not the flowering stage but good luck

Thanks for the info guys!