Hi welcome to the forum and the hobby
Unfortunately, you are possibly not going to be off to the best start as having the tank stood for a few days without a constant ammonia source will not have prepared the tank for fish. I imagine the shop will have sold you some sort of product that claims to cycle your tank in a few days, again unfortunately these products just do not work and do little for the long term health of a tank.
It would be good if you can have a read through the articles on this section of the forum.
http://www.fishforums.net/threads/beginners-resource-center.277264/
The main two to read at the moment would be the nitrogen cycle and fish in cycling. The articles will explain this in more details but what will potentially happen now you have the Platies in there is that as they start eating and *ahem* pooping, a chemical will be created called Ammonia, which is toxic to fish. In our tanks we rely on a colony of bacteria in our filters to process this and start the nitrogen cycle. In a mature tank the ammonia will be broken down quickly and converted into the less harmful, but still dangerous nitrite, which will in turn be broken down into the least harmful chemical nitrate. Nitrate is still important to keep as low as possible but low readings are manageable with water changes and if you have live plants this can keep on top of it as well.
So at present your platties will be be creating ammonia and you wont have the bacteria to make the water safe for them. This is unfortunately a very common situation new fish owners find them selves in, when I very first started I was in the same possition and it is managable but I would recommend reading the articles in the link to find out how.
Now down to your actual fish
What size tank do you have and do you know what your water is like? Is it classed as soft or hard, do you know what the ph is? If you dont have a test kit yet you can usually find this info on your water suppliers website.
If you dont have a test kit yet I would recommend getting one of these
https://www.amazon.co.uk/dp/B000255NCI/?tag= on Amazon it is about half the price of my local shop so worth getting here.
I wouldnt add any more fish until your test kit from above is showing 0 ammonia and 0 nitrite. But, long term depending on tank size and what your tap water is like there will be other fish you can add with your platties (platties do best in harder water with a neutral or slightly above neutral ph and coolish temperatures) but I would not advise adding a Beta to a community tank. They can struggle swimming in larger tanks due to their elaborate fins and often do best in small tanks by them selves.
Hope that has helped
And please keep us up to date with how things are going. I am sure that if you do run into problems someone here will be able to help.
Wills