I'm using a Stingray 140 mostly with the i84, and sometimes with the i76 or FWind1. I absolutely love the combo. Before the Stingray, I used a 71 cm wide slalom board and a 77 cm 7 ft wide foil/wing board, and sometimes a 66 cm freestyle board, but have had no desire whatsoever to go back to those boards. The Stingray is way easier to jibe, and has by far the best touchdown/breach behavior (with the old, 249 cm long slalom board coming in second). Perhaps it's not as "lively" as short foil boards, but to me, it feels like I can turn it on a dime. I'm about 200 lb / 90 kg. The 140 should also work for someone a bit heavier. For someone on the 180 lb/80 kg range, the Stingray 125 might be a better choice.

Formula boards are a great choice to get started with foiling; I have seen several guys make rapid progress on them. I think the Stingray 140 is a great "next" board that you may never outgrow. One thing I like for the i84 are the tracks - I am using the wing in C position pretty far forward in the track. That puts mast and front wing about 5 cm further forward than the tuttle box would, which gives a nicely balanced stance.

I am using the Stingray without footstraps, which works great with the i84 and the i76. But if you prefer to use straps, the inboard positions should work fine for freeride foils like the i84, and the outside positions should be fine for faster/racier foils.

I'm mostly using a 5.6 m freestyle sail and a 6.5 m freeride sail. I have used 4.7-5.2 FS sails and 7.8 freerace sail, which all worked ok. Larger sails should be no problem, the specs have 10.0 as the max. Yesterday's session was one of the best I had. The local wind meter, which has a clear fetch and is 8 meters above the water, showed 15 knots gusting to 19 knots. With a handheld meter on shore, I measured 11 knots gusting to 15. I was fully powered on the 5.6; a 5.0 would have been ok, too, but I prefer a bit more power. Here are the GPS speeds:
Still working on the jibes, so had a couple of wet ones (and a few tacks). But the Stingray has definitely helped to make progress, compared to the narrower boards. In most jibes, I just touch the water for a second or two before popping right back up. That's with a 71 cm mast, it's too shallow for the long mast.