What a dilemma! I totally understand wanting to have a lot of people in the wedding. I am having 5 bridesmaids with only about 40-50 expeted guests, but I won;t budge on that. The idea of assigning jobs is great. Maybe you can make your FI's sisters hostesses. Their responsibilities could include greeting guests as they arrive to the beach and pass out ceremony programs (if you are planning to use these).
Not sure how you feel about readings, but that is also another way you can include friends in the wedding without making them a bridesmaid. Maybe they can read a favorite love poem or spiritual reading. 2 hostesses + 2 readings + 6 bridesmaids = 10 girls included. And you can have them all sit in the front and not feel slighted. If you decie you want to have 10 bridesmaids that is your perogative, it's your wedding
I hope some of this is helpful. In the end your real friends will be happy to be a guest at your wedding. One of my very best friends to this day did not include mein her wedding and there were no hard feelings. And now that I am getting married I understand even more. Bottom line, as long as everyone important is at your wedding, everything will be perfect!