I think that would make it crowded, platies grow about 2 inch--2 platies is 6 inches, leavng you only 4 more to spare. Cories are schooling, you need about 6, and they get about 2 inches at least, that makes 12 inches. Even if you get only 3, that makes 6 inches, plus 4 inches of two new platies, plus 6 inches of old platies = 16 inches. If you have mixed sex platies, you shouldn't need to buy more, they'll have babies in no time, and your tank will get overrun with them...