Agreed on double checking for the ID.
Also, yes increase the stock numbers VERY slowly. Normally the recommendation would be no more than 50% of the current stock at any one time. Keeping the stock adjustment that small will help the biofilter adjust without a minicycle. Adding more than that and you are risking it. Also, underfeed for the next week after adding the new fish. Fish can be fasted and should be fasted regularly. More food = more poop = more ammonia. Feed less when you add the new fish and the biofilter will have an even easier time dealing with the new arrivals. Wait at least two weeks before adding more. Feeding slightly more in the second week than the first. Repeat the cycle as needed until you achieve your desired/recommended stocking levels.
(BTW, I would get at least a 1:2 M:F ratio for the platies - or go with all males. Going all females is a bit dangerous as some arrive pregnant or develop the gonopodium later and will "change" gender).