I made a "belgeglin" using saison yeast (3711 French Saison), bitter orange peel and corriander seed. Came out really well, and I'm sure the 550 will do an equally good job on a mead. I didn't use any candi sugar, though, all honey. I have no idea how much sugar you could substitute for honey before it becomes "too much," but I think 38% might be a bit much, and would agree that it might be best to go more towards the 80/20 ratio you suggested. I think you'll still get plenty of color and character from the candi sugar...it is a mead after all though, and I think the honey should dominate.
As far as carbonation goes, I think it's up to your own preference...I perfectly enjoy still mead, so I have never carbonated my meads. I suppose one of these days I'll make a sparkling mead, but just haven't felt the need yet..
Packaged: Simple Cyser '14 (Nelson Sauvin Dry Hopped!), Spiced Cider, Spur of the Moment Graff
Primary: 15 Minute APA, Simple Cyser '15
Secondary: Why do I keep this line here...?
Bulk Aging: Cocobochet, Wild Cider (2014 & 2015)
Planned: Fruitcake Old Ale, Not Just for Breakfast Stout. Otherwise, so many ideas, not enough time to brew them....