I'm not sure about the 7.5 watt heater because I've never owned one. And 1 gallon is pretty small so I wouldn't want to encourage you to put a heater in it. Maybe someone who keeps a fish in that size can give you better info.
Personally, I don't like anything that I can't control the temp. on but I know there are plenty of folks here who use smaller heaters without problems. I started out with 1.5 gallon tanks (Tetra) that I was able to keep the 25w heaters in. They're taller than they are wider so if you have room for 1 gallon, you probably have room for the 1.5 gallon. Like this one: