My first
greenhouse was connected to a south facing window of our house much like a window air conditioner. This allowed the plants to be outside, under plastic and protected from wind and rain during the day. It also allowed the heat generated by the sun shining on the
greenhouse to be passively circulated into the house through the open window during the day rather than needing a fan. A big advantage was the sunlight being reflected off the wall and striking the plants more evenly thus preventing the leggy, leaning, growth seen so often in plants grown on a window shelf with sunlight touching only one side of the plant. In your case this could be built large enough for 4 flats of starts and the flats brought in the house at night and the window closed to conserve heat. As the plants get older and need hardened to cooler temperatures prior to transplanting leave them in the
greenhouse overnight.
Cucumbers: I find the best early results (if wanting to get a head start) to be germinating them indoors, 3 to 4 seeds in a pot about the size of an 8 oz cup or a little larger and letting them grow to the first true leaf (and no more than two true leaves) and then plant out. Cucumbers roots head to the bottom of the pot as soon as they germinate. The faster they are transplanted to the garden after the first true leaf is of a decent size the faster they are allowed to grow their roots deep in the garden instead of becoming rootbound in the pot. The same thing goes for watermelon and cantaloupe and squash.
Tomatoes: My best results are planting in a deep narrow container and allowing to grow to two true leaves. I then slow the growth down by cooler temperatures at night and then plant to the garden. If grown in too large of a container a lot of the soil falls off because
root development was mostly down. This is not necessarily a detrimental thing just a waste of purchased potting soil. I have planted as young as 1 true leaf which allows the narrow container to be less deep. I plant up to the first true leaf in the garden.
Broccoli and cabbage: I sow in a wide not too deep container and then once they are of two true leaves I transplant above the cotyledons to individual containers. This helps me prevent them being leggy. Grow with cool night temps as they get larger.
Water the plants in the morning if in a window
greenhouse so they don't dry out while you are gone during the day. My father-in-law commented that if you’re a little late milking the cow things were okay but if you’re late watering a tiny tomato it is a goner.