Look online for the small heaters, they aren't usually carried in pet stores. Try www.thatpetplace.com
. I believe a company called Hydra makes heaters especially for 1-3 gallon tanks and they are cheap (less than ten bucks last time I came across one).
As for the worms, I usually replace two pellet meals per week with the worms. I drop in 2 or 3 depending on the size as some worms are bigger than others. Make sure you wash your hands after you handle them because some folks are allergic.