Don't use anything wild, they carry a whole host of bacteria and parasites which equals BAD!
IAL and Banana Leaves as well as dried Oak Leaves give off a thing called tannins, they will turn your water a light tea color (Don't worry it can be taken out with water changes) But it helps the Betta adjust to it's new home. They also have minor antibacterial properties to help ward off bad bacteria for your fish.
You can use the minnows for the cycle and then just take them and put them up for adoption back at the Petco. You can use 2-3 fish for it, just enough to create waste but not enough waste to hurt them. You can use your tap water after you run out of spring water, it will just give more food for your bacteria. If you lose a fish, throw it away and continue with the 1 or 2 that you have left until you get a reading of ZERO ammonia, ZERO nitrites and anywhere from 0-80ppm's of nitrates. Once you get those readings, you do a large water change (about 75%) and replace water and test again the next day (a whole 24 hours) and if your ammonia is at ZERO then your tank is cycled!
And yes you can use the rocks, I put them under running HOT tap water, extremely hot for about 5-10 minutes and let them cool off before placing them in the tank. It's the equivalent to boiling them.
EDIT: oh and substrate is what you use on the floor i.e. your gravel or sand, soil, marbles, glass rocks, dirt, etc.