IMHO it is best to use a vendors entire nutrient line ONLY. They spend millions of dollars developing nutrients to work together to achieve the best possible results. It is all about Liebig’s law of the minimum. When you understand this you will understand why you should not mix-n-match your own. The only exception to this rule is when you have deficiencies (it won't happen often if using nutrients correctly). There are a couple of additives I do like and use: Azos and Mykos are microzae and beneficial fungus, Botanicare HydroGuard protects roots in Soil/DWC. Other than this I use a vendors entire line only on their schedule for what I am doing. As to the strength, the vendors charts are a beginning point and for many strains or phenos it is too strong but not all strains or all vendors. So I have a system where I start seeds at 1/4 strength for the first week with a pinch of kelp and humic acid then 50% for the next and I continue to increase by 10% each week until tip burn is observed then back off 10%. This gets you to where the plant likes it not just blindly following the chart.
By all means start your seeds right in the pot they will live in. Use a root riot, rapid rooter, rock wool starter cube or plant the seed right in the soil1/2 inch deep. keep moist (not wet) and warm78°F. it is ok to use a humidity dome if you cannot keep the RH in the tent ~60%. Just make sure they have a breathing hole and just use them a couple of days at most. The risk here is the lights warm the air in the cup too much the humidity goes to 100% and you get "damping off".