Great of you to rescue the goldfish! The other two probably died of chlorine poisoning, and all the other maltreatment goldfish are subject to. That being said, welcome to the goldfish club!
. My betta tanks are NPT's and don't have BB so I can't seed it that way.
Not true. They will have some, but a small amount. Adding some substrate or decor from those tanks will help seed your goldfish tank.
As for the holes you are seeing, it is probably finrot. Goldfish are hardy enough to cycled a tank, but no fish comes through it unscathed. Even though the Tetra people advise not to use prime, use it. Even if it does prolong the cycle, your fish will be healthier for it.
While the mistreatment he undoubtedly suffered in the store and at the hands of middle school boys wasn't good, it probably won't have any lasting effects on him. This small not growing stage isn't going to stunt him. Two weeks without visible growth isn't unheard of. I bought a little goldfish from the store about 3 months ago. He must be bigger than then because my friends that see him comment on his growth, but I can't see it because I see him every day. Just because you can't see it doesn't mean it isn't there.
As for the peas, don't feed them. They are a sugary vegetable. Goldfish need leafy greens like kale, spinach, and turnip greens. Gel food will be great when you make it. I started my newest goldfish on gel foods and have no issues with flipover at all. My other two who were fed commercial pellets for years now have permanent damage.
For the temperature room temp is fine right now, but when winter comes, you're going to need a heater. Due to the deformed body shape, the digestive tract of goldfish doesn't function well below 70F. If the tank will stay above that temp at all time, that's fine, but even my 55 gal goldfish tank drops to 68F without a heater.