I've sailed a 6.5 on my iSonic 122 without any problems. I'm 95 kgs. The board just absorbs the chop and this sail size is very speedy. No doubt a smaller board will be ideal (a little faster if there are no lulls) bu the 122 will work surprisingly well with a 7.0. Actually I've just ordered a 7.0 slalom sail for mine. I use a 40 fin for this size and it works great. I can even use my 40 fin with 8.5. Some find this small but my GPS gives me an average speed of 1-2 knots more with this size compared to my 44 fin (but I need to be powered a little more).
The big difference regarding your combo is if you compete seriousely og "just" go for max fun and drag racing your local buddies. To win you need the ideal combo, but conditions aren't always ideal and so you need the "ideal" compromise.