From what I've heard, it's not safe to use any of the reptile-specific heating equipment on fish tanks. Apparently, the tape or a heat lamp can cause the tank to overheat very quickly. I personally think it's best to have separate heaters for each tank, even if it's a bit of a pain.
In the 1 gallon, you'd be fine with a 7.5 to 10 watt heater. Hydor makes a 7.5 watt non-adjustable, and Newattino (an Italian brand that you can find on eBay if you're not in the EU) makes a 10 watt that I've personally used in a 1 gallon and can recommend. I reckon you may be able to get away with using a 25 watt adjustable in that tank size if it fits properly, but you'd probably have to turn the setting on the heater way lower than the marked temperature.
The .5 gallon is trickier. I'm pretty sure you could use the 7.5 watt Hydor that I mentioned in a tank that small, but let's wait for someone with experience on that to chime in. How about floating your new girl in a small container placed inside the 5 gallon as a temporary solution? I think that's the best way to go.
(edit: OFL beat me to it!)