Unfortunately your information was not valid. There are several things wrong here.
1) Waiting 3 days to add fish did absolutely nothing. What you need to be concerned about is your tank going through the aquarium cycle (nitrogen cycle) where bacteria colonize which convert harmful ammonia into nitrate (still toxic) and finally into nitrate which is safe in small amounts and is removed during water changes. It is best to just add ammonia manually to the tank without fish to complete the cycle, but since you already have fish you will have to cycle with fish. I recommend that you read the cycling sticky on this forum and also google "aquarium cycle". It's just easier than me trying to write the entire thing out every time
. Basically, you will need a good liquid test kit for ammonia, nitrite, and nitrate so that you can test daily and do a partial water change as often as necessary to keep ammonia and nitrite levels below .25 ppm. Do not add any more fish during the cycle.
2) Do you have a heater? You say you have 2 thermometers (why 2?) but if you don't have one you will really need a heater to keep the water at 78 degrees because you have tropical fish.
3) Salt can be used as a preventative against disease, and there are many people with varying opinions about whether or not to always use salt in an aquarium. I usually don't unless I'm treating something or trying to keep a wound clean just because I don't feel it's necessary (and I have plants and fish in some tanks which would not appreciate it). Your fish should be fine with it, just remember that salt doesn't evaporate, so only add enough to treat the replacement water if you want to use it.
4) You do not need an algae eater (in fact most grow much too big for your tank) or bottom feeders. I recommend that you find some fish that you like (you can check liveaquaria.com if you want some ideas) then post them here and we can advise you if they will be compatable. Also remember that your platies (if they are female) are likely already pregnant and will have lots of babies in short notice. Especially if you have males and females....yeah, those guys breed like mad. You will need to wait until your cycle is completed until you add any new fish however, so be patient!
I hope this helped you, and feel free to ask any more questions that you may have along the way