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Okay, the following is assuming you know none of this, so sorry if you do!
After you get your tank set up, you have two routes- a nicely planted tank which will allow faster stocking, or the nitrogen cycle, which can take about a month to complete. Not going to go into too much detail for starters, have a look around the forum and see what you can find- planted tank info in the "aquarium plants" part of the thread, nitrogen cycle threads in the freshwater aquarium section. Also have a look under fresh water articles (if you want a more thorough explanation about anything there). These threads will be stickied at the top of the section, so they are easy to find.
Next, you wanna head on down to your fish shop/pet smart/tap water plant and ask for 3 vital numbers. pH, general hardness (gH) and carbonate hardness (kH). These will help us decide which fish are going to work in your water, as not all fish are made equal.
Anyways, once you get that important stuff out of the way, onto your main question. Platies and swordtails I believe can cross, which should be avoided. You can keep both species all females if you want (not sure all males). Swordtails I believe get pretty big, not too big for your tank, but I've heard they can be aggressive to fish that aren't sword tails when they get full grown. Something to consider.
Onto catfish, very general word. I'm guessing you're not looking for a 4 foot monster or a fish that can swallow fish twice it's size? Just some humor.
You'll probably want Cory catfish, small, lots of types. These will require a sand substrate for best health. You can pick up regular playsand at a hardware store, $5 will cover your tank. Wash like you would gravel pretty much. You will also want at least 5 in that tank, though 7-8 would be find since you have some extra room.
Woo, hope that covers the basics! Posted via Mobile Device