It works. Cancel after the 2nd hit of EX rekka into the fireball. There's no real trick to it, just timing. I wouldn't use that combo though. Not really worth it to use the drive cancel from regular rekkas into EX rekkas. I also go straight into rekkas after the fireball instead of adding a normal.
I have
yet to find one [DC] combo that warrants the drive use. Everything adds less than 100, most closer to only an additional 50 dmg which is pretty pathetic for a drive cancel.
It
does look hella stylish I'll give you that.
One last thing that is good about the [DC]'ing into f+AC after Rekkas is that it's a) easy to hit confirm especially if you didn't start with a normal before the rekkas (a lot of times I'll just use rekkas to punish certain moves that are too far away) and b) [DC]'ing into f+AC makes the Ex Rekkas into qcb+A much easier due to the distance.
Consider this:
s.D, f.A, rekkas x2, [DC] f.AC, qcb A, s.C, Ex rekkas x2, qcb A, s.C, rekkas x3, qcf B, s.C - (
479 2 stocks, 1 drive)
vs
s.D, f AC, s.D, f A, Ex Rekkas x2, qcb A, s.C, Rekkas x3, qcf B, s.C - (
466 2 stocks, 0 drive)
Also I've been practice the Ex Rekkas into qcb A and what I've found so far is the trick is to delay the second hit of the Ex Rekka as long as possible and make sure you cancel the qcb A kinda early (not too early)--it'll allow you to do the s.C afterwards.