I am not the right one to answer this one.
I rode the Ultra 6 m2 two and a half years ago, when it came out.
Regarding size span, yes it is possible with 9 and 6, definitely.
Not on a surfboard where it is too much, no doubt.
But on a hydrofoil you can stretch the range somewhat more.
Would never ever do this myself, but it is because I like to be in the sweetspot - HATE being too powered.
Also, for you, because you can get a small kite later, as a 6 or 7 m2 is quite big for a hydrofoil.
I dont understand fully, dont you have other small and medium kites TT or Waveboard) you can use, or are you changing your quiver and sold your existing kites?
Below 10 m2 I use my wavekites, 3½ - 5 - 6 - 7 - 8 - 9 m2, and then ligher kites in my 10 m2 and up, for hydrofoiling.
Now you know why I am not the right one to answer your queation ha haa
But my experience with other riders here are, that 9 to 6 they all regret if they have tried it, and have to sell as you can not put another kite in between now
Some here will say it works just fine, but so many different opinions - I can only rely to what many of those I know have tried, and how well I know their riding style also.
If they wanna go minimal for the sake of that, or if they wanna have a pleasnt quiver where they at a reasonably low number of kites always can choose one they will like for a given day.
12 and 9 is a great span, I also find just fine, but not 9 and 6 if you ask me. 9 and 7 will be great.