I know this is too late for you, but perhaps it will help others. Your questions are way too general for anyone to answer. You need to go on the recommendations on your seed packets, unless all of the plants you are growing are going to remain indoors.

A packet of seed will say when to start it indoors (a.k.a. greenhouse), like this (Transplants: sow indoors 6 weeks before average last frost). If it doesn't...google it.

Something to remember is that you don't want your plants in a greenhouse longer than they need to be. For one important reason is that some plants do not transplant well when too large or past certain time frames in their growth...this is a big reason for not starting too early.

Also, greenhouses are breeding grounds for disease and pests. Always a battle against... time, light, moisture, air circulation, temperature, disease, and pests. These pressures increase over the continued use of the greenhouse and with the numbers of varieties of plants you choose to grow.

Grief - a bit discouraging there. Anyway, my cold (12x10feet) greenhouse takes up a full third of my tiny urban garden...and I use it for everything. Storage, potting and, of course, growing. In truth, growing a lot of hardy trees and perennials, the greenhouse could easily be empty in winter - it's main use is to protect my pots from rampaging birds and small mammals - but does come into it's own for the spring sowing. A brilliantly useful adaptation is automatic venting - By late June, all the doors and windows are open all the time.

I have not found it to be any sort of extreme disease sump - no more so than any other part of the garden.