The pH of one of the major river systems they are found in, when measured down stream by those collecting cichlids in the Rio Papaloapan, was 7.5 to 8.0. Most of those Mexican waters are alkaline. However the platies especially are found more in tributaries and backwaters. Some of those even have a peaty bottom under all sorts of water plants. These are not saline waters.

Platies are often found further up stream. While these are whitewaters as we describe South American fishes, with a fair mineral content, these are still not as mineral rich as some of those water supplies in the US which aquarists have to put up with. [Yes I know that Mexico and Belize are in North America. :) ]

Aquarists trying to keep wild platies from Belize have had trouble with all male batches of fry according to Wischnath in his Atlas of Livebearers of the World. That is getting close to the Mosquito Coast and I wonder if part of the problem is that those waters may be lower in mineral content and most western aquarists don't have that kind of water.

The commercial platies (or platyfish in some scientific articles - in reference to any early genus they were placed in) have been crossed with  various swordtail and platy populations and maybe even with variatus, so we very tentatively can even speak of a species with those commercial strains. None of them are marine or brackish though.

As secondary freshwater fish, I don't think you hurt them at all with that much salt. But, despite the paucity of mineral content readings for those rivers, when I went searching, I also don't think that sodium chloride is at all needed by the platies. If you wish to put a bit in, there's probably no harm, just as there is probably no harm in putting a little salt in with guppies, unless your water is really low in minerals. You may limit some of the types of plants you could raise with them, but there are still several plants (hornwort, Java moss, Java fern) which will tolerate that salt.. :)