Guppies certainly like the same water as platies, and are suitable for the tank you have. But just to warn you that with male and female platies the tank will soon be full of baby platies. There won't be room for much else in a month or two.
And as Fluttermoth commented, you have a bad ratio of males to females. With mixed sex livebearers, there should always be at least 2 females for every male. Male livebearers exist only to eat and breed. They chase females constantly. Your poor female will be chased by both males; she will never get a moment's peace. The reason for having more females than males is so that every female gets time out while the males chase other females. So the first fish you need to get are 3 or more female platies. But then you will have 4 females giving birth every month.
To be honest, in a 55 litre tank I would take the female back and have just male platies. That way you won't be inundated with platy fry. And the same applies to guppies which also breed like rabbits.
But it seems as though you are doing a fish-in cycle so you can't get any more fish yet. At the moment you will be doing daily water changes to keep ammonia and nitrite at zero. If you add any more fish now, the levels will just go up faster. You need to wait until both ammonia and nitrite stay at zero without having to do a water change for 7 days in a row before you can think of getting more fish. This will take a few weeks yet, plenty of time to decide which fish next.
