Running the tank for a week will not be enough time for it to cycle and this could cause all your fish to DIE..dead..bite the big one etc. etc. etc. Sorry. It's almost 3am so I'm a little tired

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LOL, such varied terms for death
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for jgilley84
mature media is filter material from a filter that is currently running and has been for at least several months. The mature filter media/ material has beneficial bacteria in it that keep the water clean. If you can add some mature media to the new tank the filters will develop quicker and you will have less problems with ammonia and nitrite developing in the tank and stressing or killing the fish.
If you don't have any mature media to add to the new tank then the tank will have to develop the bacteria over a course of a month or so.
If you got the tank from a friend and it was set up and running when you got it. And you emptied the water out, transfered it home and set it back up straight away the filters should be ok. However if the tank has dried out or the pumps have been turned off for more than a few hours, chances are the good filter bacteria will be dead and you will have to start the filtration cycle from scratch.
To set the tank up just use any sort f aquarium gravel or sand. the choice is yours depending on the filter. If the filter is an undergravel filter, (plastic plates that sit under the gravel) then you will need course aquarium gravel. If the filter is a sponge or power filter then you can use sand or gravel.
Floating plants like water sprite (Ceratopteris thalictroides) are good for livebearers. But most true aquatic plants will be fine. Most people have tall plants at the back and shorter plants on the side and front. Your choice as to how you want it decorated.
As to what numbers of fish. If you want to breed them get females and don't bother about males. Female livebearers are just about always pregnant when you buy them. They also carry sperm packets and can have multiple batches of young without a male being present. The males will pester hell out of the females as well. If you do want a male get 1 and keep him in a seperate tank. When the females are no longer producing any young (6-12 months time) you can put him in with them for a couple of weeks. Then take him back out.
Mollies, platies and Guppies generally don't eat their young. Swordtails will eat their own and other fish's young.