well, I don't go by the inch per gallon rule. its out dated and not the best way to tell in my opinion.
some things to consider is the fish size, activity, how much waste they produce, etc.
I am new to platies. I really don't know much about them, oviously they are not complicated fish because all of the caresheets I can find are very basic.
hoenstly just right now two of my platies don't seem happy. they are all young females so the are not crowded. I have had these fish for two days, and two of my platies are constanly swimming up and down the corner of the tank rubing their noses
this tank is not cycled, Biospira will be here tomorrow. but with only having them in it for two days I doubt that is why they are being wierd.
the tank was running for a week before adding the fish.
cycles can't happen without a sorce of waste... so no there is no way it can go through a cycle before adding fish.
I know all about cycling. this makes my sixth tank that I have cycled. its my first time using biospira though! I decided against cycling with the platies.