03-29-2013, 07:38 PM
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Welcome to Tropical Fish Keeping forum.
The best way to deal with the temperature in transit is to get yourself one of those picnic-type coolers. They come in various sizes and are not not expensive. Not only does this maintain a steady temperature, but it adds darkness which is almost as important. Fish in transit will be more calm if it is dark, total blackness, around them.
Tell the store the length of time involved. Some of the better stores will increase the bag size, add extra oxygen, etc. I would not delay getting them home any more that is absolutely necessary. Fish secrete pheromones and allomones and these can build up in the water, along with the ammonia of course. And molly are very highly sensitive to ammonia at very low levels.
To the question of swordtails and molly, in a 20 gallon the answer is no. Swordtail requires a larger tank as it is a larger fish and needs space to swim. Check the profile, click the shaded name. The Common Molly also gets a decent size, and in a 20g high I would not have more than 3. All male is fine; one female in with one or more males will mean hundreds of fry regularly, and getting rid of them is not always simple.