My guess would be with your ammonia being so high even after water changes, it will be tough to keep fry from dying as they are much more sensitive to water conditions than adults. Also, in a community tank, it is normal for most of the fry to get eaten.
Really, this is ideal, because (a) with 7 platies, as long as you've got a few females, you could populate that entire tank and then some fairly quickly even with most of the fry being eaten, platies are pretty prolific breeders; (b) Since you are just at the beginning of a fishy cycle, it would be very detrimental to have a bunch of fry to survive - essentially you would be adding 5 or 10 fish to your tank. It would cause the cycle to take longer, because now the bacteria has to try to keep catching up to a higher bioload, plus it will just make the ammonia spike even higher.
At this moment, while cycling, I think the best thing would be to let her have the babies and don't do anything; if some survive, ok; if not, then all the better since this is not really a great time for you to be adding fish to your tank.