I have gotten two sets of Sky Guardian: one from Crius + Living Flame, the second from Flamestorm Finery level 19 + Spectral Captain level 20 (gave Sky Guardian level 20). But you're far more likely to get Wind Monarch's Robes or Rocfeather.
And yes, you can only get the "plus" version from chests.
AFAIK level has ZERO effect on the type of resulting armor. The only thing it does is that the _level_ of the resulting armor is the average of the two source armor.
The only things I know of to manipulate the _chance_ of getting a certain armor are:
more stars in is bigger chance of more stars out
fewer elements in -> a smaller pool of result armors to pick one from by the fusion process (unless you're using Legendary mono-element armors as source, this will conflict with the first point in most cases)
I got Sky Guardian by combining/fusing Torchflame Mantle with Monstrous Garb. It might work for you, too! P.S. I was really excited when I got Sky Guardian. I was expecting to get Molten Shroud or something.