I have a 7.5 w heater in my 1.5 gal. It's meant for glass bowls/tanks up to 3 gallons.
Only downside is that the temperature it heats to can't be controlled. It only heats up the tank an additional 5-10 degrees. On some packaging of the heaters, it says if it can be used in glass/plastic/acrylic as well as how many gallons it will work for.
Do they even MAKE small 7.5 heaters like that to where the temp. can be controlled? O-o
I don't own any tanks under 2.5gal, so I'll give you my personal experience based on what I know. I have plastic and glass. I use heaters in both. I had the same concern when I moved to smaller tanks and I was assured by these experienced forum folks that using a heater in the plastic tanks was just fine.
I have used the smaller heaters that you cannot adjust. I don't care for those because I've had bad experiences. They skyrocketed the heat to over 85 degrees and I almost cooked my fish. However, there are folks on this forum that use them and love them. So through experience you'll find which works best for you. Personally, the best luck I've had is the adjustable 25W. I have them in all of my 2 gal tanks. Pet Club carries them for about $14. I have them in my glass and plastic tanks. I've even used 50W adjustables in my small 2 gals and just turned them down. They've worked great too. I prefer heaters I can adjust. Hope that helps.