Yes you are incorrect. Most of these 12mm flexis end in 15mm compression or 1/2" female unions, some "foreign" ones will end in 3/8" or a.n.other. A good way of connecting them is with the appropriate size of male iron, eg 15mm compression to 1/2" male iron; these have a nice wide flat face and provide the perfect surface for the washer.
The problem with filing an iso down is that at what ever point you stop filing, the nature of the spiral thread means that at one point the wall is very thin - some makes are better than others, granted, and I have been know to do this - you just have to be careful!