1) you can start LSTing a plant as soon as it pops out of the soil.
2) you can add mycorrhizae that way, you can dust them on the outside of root balls, or add them into your medium when you mix it up. I don't think it really matters, to be honest.
3) do some research on what trichoderma eat before you add them. They are mycoparasitic, which means they are, well, parasitic to other fungi. They could become parasitic to the micorrhyzae.
I hope this helps.