Hi there, first off - the Sky SUP was designed primarily as a good SUP foil board, which is also our best learner board for Wingfoiling. And then we added the WS option as we found it worked quite well as an advanced windsurfing foil board too.
You can't use the Flow front wings on the Aero fuselages unfortunately for now, next year should have the compatibility people are looking for. The mast can be used for both, a 75cm is good to start with for both sports, but generally a 90cm is better for both windfoiling and wingfoiling once you have the hang of it (at least for most conditions, in waves it's sometimes better to have the 75cm for wingfoiling in lower depth conditions or walking out through beach breaks etc?)
Indeed the Aero and Aero High Aspect are better for Wingfoiling, which they are designed for. But the smaller Flow 1000 is also a great foil to use for wingfoiling, you just need to change to the Flow 90cm Fuselage and 215 Flow back wing, all available separately.
The Aero 2000 is built for SUP foiling and Wingfoiling. So you could windsurf with it, but it's a huge wing so probably better to go to a High Aspect 1250 or max 1500. Will still obviously be slower than a Flow Foil, as these are built for the higher speeds that Windfoilers achieve/want.
re Mast further forward in windfoiling - it is good when using an older shaped windsurfing board, instead of the classic tuttle box position. But the longer Flow fuselage (90cm) and longer gap between mast and front wing, already improves this. On the Sky SUP, the boxes as you mention are well forward, so no need to go too far fwd.
Yes, the Sky SUP 6'11" is mainly intended for light winds/heavier riders/learners. So below 20knots, all fine, over that it may be a bit of a handful. Hope that clears up your questions, feel free to ask more if needed?