How nice to share a greenhouse with friends! For most seedlings they do not need extra fertilizing until you see two true leaves. The first set of leaves are part of the seed itself not true leaves.  You really want young seedlings to grow slowly until it is warm enough to set them outdoors so I feed with a half strength water soluble fertilizer if the seedlings look pale and need a shot of food.  Congrats on growing from seed. Such a wonderful adventure. Keep growing, Marianne Binetti