from what i understand grow big isn't much of a fertilizer, it has trace amounts but it is packed full of little microbes and other bacterias that are good for the plant and help make it healthy, it also helps the roots uptake the nutrients more efficiently,(I could be wrong, I've been known to be at times) I personally use Sea Green but it's basically the same thing... worm casting, and other odds n ends. but yea i def don't feed untill they are transplanted into their perminant pots. as for watering i wait untill the soil is dry but not so dry the sides start forming a gap between the soil and pot and just water untill i see water draining. After awhile all you'll need to do is pick up the pot and have an idea of how much is needed.
As for the switch into flowering yes just mix big bloom and tiger bloom in one gallon jug I also like to add 1-2 tsp. of blackstrap mollassas (They love it)