For goldfish rearing, yes you can overstock your tank with goldfish, if you got superior filtration system that able to neutralize ammonia as fast as the fish produced(Mature fish tank), and also u got time to do more water change, filtration only to remove dangerous ammonia but water change still necessary.

In Our country heater is not required. Goldfish able to live in quite low or high temperature. Just to make sure the temperature did not crash and the fish will fine.(Temperature crash, maybe suddenly pour new water in fast pace and cause temperature up or down in short period)

Before start rear goldfish, learn about cycling fish tank water. Else you need to perform water change almost everyday until the fish tank water not smelly. Smelly fish tank water usually meaning high ammonia.

Sinking pellet always better for goldfish. No feeding high portion of food in single feeding. To increase fish size, feed fish by dividing to more time of feeding, but less portion each feeding to avoid swim bladder issue.

Cheers.