You can save the 8-14-9 for later. That will be more useful during the flowering stage. I would just use the 10-8-7 you mentioned in the first post during the veg stage unless you get some other brand that has more nitrogen (the first number).
Here is a good, SAFE way to do it:
1) Wait until 2-3 weeks (depending on growth) before doing anything.
2) Then fertilize 1 one plant with 1/4 of the dosage, see how it responds.
3) Next watering, if it's doing well, up the dosage to a 1/2. At this point, you could start the rest of the plants on the 1/4 dosage since you know that's safe.
4) Continue this until you reach the desired dosage.
I would ramp up the dosage to no more than a teaspoon per gallon. Anytime I make changes to the nutes, I always start with one plant as a trial. That way, if you mess up, you only lose one plant and not the whole crop. Definately worth doing if you're not sure what your plants can take.
BTW - In the future, I would avoid soil with built-in nutes. It just complicates things.