First, hello and welcome to the forum.
Unfortunately, 5 gallon isn't very big and really limits what you can have. The people that told you 1 or 2 are closer to correct. I have a 5 gallon tank with nothing but one male betta in it. The general rule of thumb for beginners is 1" or adult fish per gallon of water. That would pretty much limit you to about 2 or up to 4 small tetras.
You mention adding fish a week after the tank is set up. Do you know about cycling? I'm sure it was mentioned in the instructions that came with your tank and filter and maybe even by the people at the fish store but there is a big difference in truely cycling your tank and simply letting it run for a week before adding fish.
Cycling is the process of developing the beneficial bacteria colony to process fish waste (ammonia). First bacteria develops and processes ammonia which is toxic to fish, transforming it into nitrite which is also toxic to fish. As nitrite becomes present, a different group of bacteria forms and processes nitrite, transforming it into nitrate. Nitrate is toxic only at high levels and is removed when you do you weekly water changes.
In a cycled tank, ammonia and nitrite should always be at 0. Anything else means you have a problem and that your fish will be at risk of not only ammonia and nitrite poisoning but also by other diseases such as ich that are triggered by stress in our fish.
There are 2 ways to cycle a tank, with fish and without fish.
Here is the link to the thread on how to do a fishless cycle (obviously the easiest on the fish as there aren't any when there is ammonia present). It is also the easiest on you as you aren't constantly having to do water changes to keep the toxins low.
If you choose to cycle with fish, you should choose only a couple to start which in your case is almost going to be fully stocked. You should get a good liquid master test kit (you need this regardless or whether you cycle with our without fish) that will test ammonia, nitrite, nitrate and pH. Once you have added your fish, test ammonia every day. You will have to start doing partial water changes of 10 to 25 percent on about the 3rd day to lower the ammonia level. On about the 3rd day, start testing nitrite. The same applies to water chagnes there too. After about the 7th day, test for nitrate to see if any nitrite is being processed. You will also want to test your tap water so you will know if you have any toxins in the tap water that could cause problems. Its not uncommon for that to happen.
You want to keep both ammonia and nitrite under .25 ppm. This usually will require daily water changes until enough bacteria develops to handle all the waste and keep the levels at 0. It will probably take about 3 or 4 weeks. During the time you are cycling, feed lightly. Less food means less waste and less water changes. Fish can easily go for a week without food so feeding only a small amount once a day or every other day won't hurt them.
If you get platys, get all males. You won't have to worry with fry and more overcrowding. Females will have another batch of fry every 28 days and even if you let nature take it's course and the other fish eat them, some will still survive and in a couple months you will have 15 or so fish and major water quality problems.
Sorry to get long winded on you but just wanted to help you get started in the right direction. Good luck!!
