Hey inkslinger. When to start dosing (and how much) is tied to the number of plants in the tank. I don't think you should think about that too hard. Instead, I think you should start getting plants as soon as you can, then immediately start with a dosing schedule like EI.
You might want to think about what kind of tank you want: sexy stems, long term easy scapes with moss and anubias, etc, then think about light and CO2 requirements. But I think you should just get the plants you want and keep inching that CO2 a little higher into yellow, slowly. Regular dosing and lots of CO2 will grow lots of plants even with ambient sunlight. More light just makes it faster/harder/more time consuming.
Once the plants are growing, you know they're taking up nutrients and if densely planted will quickly take up ammonia and stuff. At that point you have a huge saftey net to add more fish, though you should still do it slowly and watch the tank. I've stocked whole schools after my first big trim and the fish did great. Acclimate slowly, etc, of course.
Hope this helps!