Can anybody tell me how to know which of my fish are mollies & which are platties ( or should that be mollys & plattys)? I'd hate to buy my male a couple of girls of the wrong species. He's young & doesn't need the confussion!

Hmm a little bit tricky here cause you didnt post a picture of your fish. BTW are you sure your fish is a male... LOL..... Just kidding. I think there's a distinct difference between mollies and platies in color variation and physical appearance. There's has not been a color clash between the 2 groups, however it's better to identify it based on the physics. Platies generally has shorter stout deep bodies compared to mollies & other livebearers too. Somewhat look stunted. Since it's a male it'll be easier. I only know of 2 variations of mollies i.e. the sailfin and the double sword or aka black(I think). The double sword variation has two elongated tips of its caudal fin which looks crescent like (not like the double swordtail) and it only comes in black color giving it's name the black molly. The sailfin mollies comes in quite a variety of color. The distinct features is it's dorsal fin that can be spread out like a sailfin during courtship period which is an interesting phenomenon. It looks just the the merlin's sailfin only miniscule in comparison. The based color can be silver, orange, grey, black, calico with some spreaded spots. So I hope this help.