You’re plants appear to be stretching, that’s usually from not enough light intensity. You could probably lower your light some for now. Then just bury the stems a little when you transplant. You should also place your clear cups into colored cups to keep too much light from hitting your root zone.
I usually wait until the plant is about as big around as the container it’s in to transplant. But a little sooner or later isn’t a big deal. Nutrient timing will depend on what you’re using for growing media. An inert media will need nutrients as soon as cots start to yellow. A well amended soil can feed for quite some time, some even the full length of grow.