I don't think there will be any gap between the ultrasonic and the F111. You can use the 7,8 on the US and the F111. I used the north RAM 8,5 on the F133, this was a good combination. I only replaced the standard fin for the C3 sting II 48. this one gave more lift in low wind conditions.
The 8,5 I will also use on the Ultrasonic. The advantage is that I can always go with a bigger sail in the future, while the F133 is at his limits. The same story for your 7,7. You can use this also on the US.
I have a 7.8 North S-type I will use.
Hope this helps,
F111, F93 ('11)
North RAM F11 8,5
North RAM F12 7,8/ 7,0 / 6,3 / 5,7 / 5,2
North S-type 9,5('11)