Seeds sprouting has nothing to do with the ph.
The Big Bud should be great in bloom, it is PK boost. I think you will probably be able to pull a decent harvest down using the floragro throughout and then just add teh big bug from weeks 3/4 to week 6/7 (read up on how people apply it, basically during flower/bloom the plant suck up ton of P and K so you need to add them.)
The PH pen is pretty necessary in coco, they are stupid expensive at the hydro stores but you can get cheap ones on ebay/amazon for around 10 buck, sometimes they even include a PPM/TDS type meter. Nutrients will swing that PH away from whatever it started out as....however, you can sort of do some educated guessing. Many lines will bring your PH roughly to where it needs to be if you follow the direction. Try and figure out what exactly the PH is of the starting water, then just google up people using your product and how much it swings the PH and go from there. My water starts at 7.5-ish, recommended nute amounts bring it down to 5.5-ish, just to give you an idea of what sort of a swing you are going to see between the starting water and what happens when you add stuff.