Crayfish are cannibals, so in order to keep multiple specimens you will need a large tank - larger than you have. Just stick with the one, or if you want, you can try keeping a pair. You can sex them by looking at their undersides - pictures are available on the Internet (easier for you to look them up than for me to explain).
As for keeping crayfish with fish - there are some options you have. Like you said, anything that sleeps near the bottom isn't going to last long. In my experience, barbs make excellent tank mates for crayfish. Danios do too. Also top water fish like golden wonder killis and African butterflies are safe.
Crayfish live in both warm and cold climates, so temperature is not very important.
Just make sure you have a good top - they are very good at escaping, though in my experience they only try to escape from small tanks.
Gravel or sand? Doesn't matter.
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