just about any newt species will do fine under/at 19C all year long so no heater needed at all. depending on the species size and type you could house 1-4 in a 10 gallon. other aquatic salamanders like mudpuppies, axolotls, sirens, would most likely be too large unless there young

here's a list of a bunch of newt species, some with detailed caresheets Caudata Culture Species DB

and also the caudata.org forums are great for info
http://www.caudata.org/forum/