One of my tanks is a 5 gallon acrylic Kritter Keeper and it does fine with a heater in it. My other 5 gallon is a Galaxy 5 and it was given to me by my daughter-in-law, and has been heated for a few years. They make a lot of acrylic tanks these days so it seems that it wouldn't make sense to make them impossible to heat. It always better to be safe rather than sorry though, so good question. Good luck with your tank.
Nope it doesn't melt, I'v had a heater on an acryllic tank and it doesn't melt, doesn't seem to do anything when it touches the acrylic either, probably because a heater usually isn't on long once the tank is heated up.