Hello friend. It sounds like you need to do an epic amount of reading.
You can get Epsom salts at just about any local grocery or walmart/target type store.
You transplant plants when they get a good root on in there current container, but before they start to slow down an loose color.
And yes, they idea behind super soil is to have a big enough container that the plant can go all the way through flower with just the nutes in the super soil, located at the bottom on your pot. Usually best in at least a 5 gal, more like 7, 10 or bigger.
The get your dank on, all you need to do is read and learn. Cruse the stickies. Once you have read the stickies and understand what they are telling you, then you will have a good start on your journey.
Live Love friend,
Oriah