The best reference for when you should plant individual seeds is the back of your seed package or the website that you ordered them from.
Lettuce + Kale
Use soil blocks in to produce healthy seedlings that can be started in close proximity. Check out my newest tutorial to discover how to mix, pack, and press soil blocks of various sizes easily.
Other Winter Squash
Rhubarb Companion Planting Guide
Soil Blocking: A Simple Beginner’s Guide