Hmm, for us riding waves, some choose to have a 5 and 6 and 7m2 (if possible).
Because these are the sizes used most of the time, and a huge size difference from 5 to 7 actually, so having a 5 and 7 as many of us do, is not that good and a huge step, if you are experienced (= spoiled) and want bullseye kites.
But that was a bit away from the question, sorry...
Would also advocate you take a 9 and 7, eventhough you ride TT.
As a 6 is really small, and you will drop like a rock, when jumping with that size.
You can NOT use your EVO 6m2 as a smaller kite to go with your new Rebel, as it will be the same size, maybe bigger, than a Rebel 7m2.
The older EVO is extremely powerful, at least one size bigger than its physical size...
Another advantage by going 9-7 for you, is if you decide you want a small kite one day in the future, you can add a 5m2
(Instead of having to sell the 6, and buy a 7 and 5 that is)
If you go 9-6 you might be able to use this as a "survival" simple quiver, true.
But if you go 9-7 you got a really good setup that feels good when changing sizes, without having an "awful" gap like a 9-6 gives everyone I know (both TT and Wave kiters), and you will even have the option to get a smaller kite later, IF you want that, or not
My experiences from what I see happening during the years, depending on what sizes different riders (and weights) start up with, and how they end up and feel about it during these years
If you want to keep your 6m2, it is another ballgame, and you might have to think very different, maybe just buy a 9 or 10m2 and thats it, and keep this as a two kite quiver.
Remember, the old 6m2 EVO should be compared to something around 7-8m2 IMO, talking Rebel or other typical allround medium power kites.