Whatever heater you choose be sure to test it for a couple of days when you can observe it. I have seen a 10 watt spike and kill someones fish. I would use the 1 gallon as a hospital tank. Here is a cheap item I am using as temporary tank for about a month that is only $10.00 and can be found in the food storage area of Walmart:
Provide lots of plants for the baby especially floating on top. Be sure to put some air line tubing to prop the lid a little to let air in. This is a great inexpensive filter I am using (free shipping you will need two) you need an air pump and the T valves that control the air pressure and a tank to share the air with so the air pressure is low and even. You will get good water quality for both of your fish. You can PM me if you want pictures.
Assuming you do this I like the 7.5 watt Top Fin Heater in a non breezy room I can get 80-82 degrees from it. Sold at Petsmart $12.99. If you have a Meijers in your area they sell Tetra 5 gallons for $10.00. Nothing comes with them. A 10 watt Top Fin works well with these. The filter I suggested works great on up to 10 gallons.