I am no expert but I have found the easiest way to sex goldfish is whether they have breeding stars/tubercles on their gill plates and pectoral fins. Adult males have them and females do not.
With regards to venting, females protrude and males do not. Based on your photos I am guessing:
1. M, 2. F, 3. F, 4. M, 5. M, 6. F
Females are also rounder in shape and are chased by the males relentlessly during breeding. There are videos on YouTube that can better elaborate my Cliff Notes explanation. Hope this helps.