Ah well, if it was a fish shop that said that, they don't usually believe in or understand fishless cycling. He has probably been taught that first fish should be hardy ones. Full stop. He's probably never been told that such things as fishless cycles exist, he just has his check list which says "if the customer is getting his first fish, recommend fsh from this list".
The vast majority of customers go into a shop, say to the workers I have a new tank, I want some fish, what do you recommend. They don't often get customers who have done a fishless cycle, or have read up on fish and know what kinds are on their shortlist. I've overheard some interesting converstions in shops over the years. Like the couple who were pointing at tanks saying
'can we have some of those'
'no not with the fish you already have'
'What about those'
'no they'll get eaten too'
'OK, what fish can we have then'
'These will go with the fish you already have, and these and these, which of them do you prefer'.
That is the kind of customer shops usually have to cope with.
Or the couple who said to the worker - 'we got some fish from here a couple of months ago and they were this big. Now they're this big - why have they got bigger, you never said they would'
[Have I mentioned that the fish in shops are juveniles and that's why you should always check what size they'll grow to - that's why the community creator is useful]
Oh, and after all that waffle, platies are actually 'hardy' fish in the fish-in cycling sense. They like hard alkaline water. But they do grow quite big which will reduce the numbers of other fish you could keep with them.