I think the answer depends alot on how much higher wind you wold be out in.
I think the Carve 144 will do pretty well with the stock 48 cm fin in normal 6.5
If the wind gets to the point where you are really overpowered on your 6.5 m2 rig,
(in reality 5.5 m2 conditions) then the combination of too much fin and being overpowered is going to create some issues.
Also whether you are sailing on fairly flat water in 20 knots, or somewhere with voodoo chop will also become part of the problem.
How wil you know you truly have too much fin....?
When the board starts to "tailwalk"! If you've never experienced "tailwalking", when it happens, due to too large a fin, it will be a bit scary. The board gets up out of the water on the fin lift and the board gets really hard to control directionally. It sort of
"shakes it's head" like a hooked fish and you can't control it.
Changing to a smaller fin, with less lift will settle your board right down.
Other things you can do to keep sailing with too much fin and grossly over powered are:
Move the mast foot forward a little to engange more of the bottom of the board in the water and increase the drag. This works, but you may get a very rough ride as the board may get alot of fore and aft "pitch" slap and be also kinda hard to control.
I'd look around for a decent used 42 cm fin (maybe someone you know has a stock fin from a Carve 133 or 122 that you could try, and if it works, buy it as your small fin.
Or look in the bargain fins bin at your local windsurf store and see what's available.
Another solution would cover two bases.
Get a smaller weed fin. Then you get the same or slightly more area in a fin that sheds weeds, and usually wed fins are not a prone to "tailwalking".
Hope this helps,