In short, all kites smaller than 10 m2 works just dandy for hydrofoiling, so you can use "whatever" kites you got in this range.
Only when you go to bigger than 10 m2, you need "lighter" kites, as otherwise you will experience they suddenly "drop" in lulls.
Possible to ride with these when learning, but you will soon find out it is a PITA...
But your 13 m2 Edge is actually a really good and light kite, compared to many other "normal" kites, so will work well
If you plan on using smaller than 10 m2 kites, then you dont really need extensions IMO.
They are good when you are on the verge to not being able to foil, but mostly with bigger kites.
When below 10 m2 two different kites will give you WAY more windrange, than normal and longer lines on one kite
Having said that, extension lines cost almost nothing, so should be in every non racers quiver