weak seedlings do not grow with others

I would say the issue with the plants in the coco is nutrients. these need food from day 1 basically. At very low doses but feed with every watering. You should have a EC of 250 ppm or so for the first week, mostly nitrogen and phosphorus and calcium. One issue with coco is calcium. Was the coco buffered? Did you rinse it with a calcium solution before planting? Coco sucks up a lot of calcium, and holds tightly on to it. Only once it has grabbed onto all the calcium it wants, will calcium then be available for the plant to use. I always felt feeding plants was over stated, especially in the beginning, and I started a seedling in coco. It popped very nicely but after it pushed out its first set of leaves it did nothing. I thought it was a dud, so when I was watering my plants that were in full bloom, I decided to just drench the plant with high PPM water. In a few days the plant started to take off. It was then that I realized that it really is true, that there is no food in coco. Peat will have some nutrients in it which will feed seedlings for a few weeks, which is probably why these are growing significantly better. I'd give it a good feeding, about 250 ppm with a nitrogen and calcium solution and see if it starts to turn around in a few days or so. I wouldn't be too concerned about the temperature swings. As long as it is consistent, the plants should be okay. If you are going organic, a mix of liquid fish, kelp meal and some hen manure diluted in water will give a good readily available source of nutrients to get them going a bit.