yes you should be able to vent them at that size, you can take them out and look at their privates, females will look like Oo and males will be oo, but really if you are aiming for a breeding pair your best bet will be to get several more of them and grow out a group of 5-6 if you can as this will give you the best possible chance at getting a pair that will not kill each other and since they are so rare in your country you shouldnt have any problem selling the extras for a pretty penny once they are grown out more, and yes females can have spangles in their fins however they wont have spangles on their bodies while males will. but at that size they are notorious cross-dressers so only way to tell for sure at that size is venting