Mollys do not require a heater (contrary to popular opinion). Go to
http://www.fishbase.us/search.php and read particulars about thousands of fish species. They do however need a LOT bigger tank. The Goldie could probably stay with the Mollys if you had a 30 g or (preferably) larger tank, neither need heaters. Those algae eaters are flat
MEAN and you should find someone else to take him. He'll harass your other fish like crazy. A Betta could live in that 5 g tank you have alone or with a few mystery snails. Don't mix male and female Bettas though because the boys will kill the girls. But a Betta needs around 80 degrees.
A male Molly has a very pointed fin underneath at the bottom, a female has a rounded fin underneath. I can't really tell from any of your pictures.
Now on to read the other answers
Edit: If the Goldie is panting your tank probably isn't cycled, or cycled well enough for the added fish load. Nitrate poisoning will make them do that as will ammonia poisoning. Join up at AquariaCentral.com Tons of good information.
Also-yes, you can find used tanks for cheap or free. I've got a 130 g with stand and a couple 55 gal tanks sitting on my porch that I would GIVE away if someone would come get them.