The only tankmates you can have in a CYCLED 5 gallon are a snail or shrimp. (I highly recommend shrimp, snails are cute too though)
Also do you know if they're all males or mixed sex? If I remember correctly platties breed easily.... and if you have any females they're likely already pregnant..... If they have babies your betta may hunt them and get boalted or sick.
Did you quarantine the tank mates? If they were kept in the pet store tanks they may have parasites and it's always recommended if you get tankmates to quarantine them for a few days to make sure that they're completely healthy.
I'd take them back honestly, especially if you're new to the hobby because it can be a lot to deal with especially if they make POW sick.
(I LOVE LOVE LOVE Pow btw! <3 He's GORGEOUS! <3)
That or you could get a ten gallon just for platies and then you could have more and not worry so much about them breeding or making your betta sick.