I have no experience with 'real' greenhouses, but I have some experience with the little plastic zippy types.
Here is my first one, about 6 x 6 x 8 feet. Looked cute, zipper broke almost immediately, the shelves were flimsy but I used nylon zip ties and attached pieces of the wire closet shelving to make stronger shelves. I used two concrete pavers (visible in the first photo) on the lower shelves to keep it from tipping in the wind.
Then my dog found a chicken at midnight and the only safe place I could think of to put a chicken 'temporarily'
was the zippy greenhouse. It worked out just fine for about a year, chicken and plants co-existed except for the plants that were tasty.
When the chicken was finished with her 'temporary' digs, here is what the zippy greenhouse looked like.
So I got a new one...zippy greenhouse, not chicken.
Since the weak points of of the old zippy were 1)the flimsly plastic and 2) the plastic connectors, the new zippy came with a pleasant green colored mesh-reinforced plastic. Also, I took a few days and a lot of patience to wrap/cover each plastic connector with outdoor duct tape.
Before I was finished with this job, a big wind knocked 'new zippy' over, so I built a heavy wooden frame for the base. (Note: there is no wooden 'floor' on the zippy; I was doing the work with the zippy up on my deck to save my back muscles.)
For your original question about seed germination, I am a firm believer in using straight vermiculite - learned the idea from Jonna Sudenius. This is my version. Had great success.
