Sorry to cut in on the "ask our team".
If you intend keeping your iS111, then the iS131 would be perfect change up board for 8.6m and up to 10m. XL slalom really becomes fun when maxed out on 10m in light winds.
If you intend getting rid of your iS111, then go with the iS121. This will be good for 7.5m to 9m so will kinda tread into both iS111 and iS131 territory.
Yes, a 6.8m is way way to small for iS131, will be feeling bad
Remember, have 2 or 3 fins per board and you will nearly double a boards range. A good 44cm to 47cm on your iS111 and it would be good to go with a 8.6m.
One of my favourit combos is the iS111 and 9m/43cm fin. Im 105kg and it works from about 15k, nicely powered up