I have been racing formula for 6 years at the local level. On a few occasions, I have gone down to a 6.5 on my F160 and F147 a few years back in races with winds to 30 knots. I am an older guy and when necessary I go into survival mode if I must. I have used a 7.5, 8.4, 9.2 & 11.0 while racing on my formula board, but for fun sailing, I will jump on my iS 111 as soon as it's blowing 15 knots.
Before I bought the iSonic a year ago, I stayed on my F160 for free sailing on anything under 20 knots and occasionally used my 8.4 and 7.5 because I did a lot more reaching.
As for fins, if you are racing upwind and down wind, you can stay with a larger fin. I found that the Drake was a handful when it got windy, so I almost alway use a Deb R13 70M which gives me a lot more control. I have a custom Curtis 68 that I use when it gets really windy but it doesn't point well unless I have a lot of speed. If you are going to do a lot of reaching, then a smaller fin - 60 or 65 should would work fine. Going below 60 does't seem necessary. A lot depends on your weight. I weigh 79kg.