I use stock ESC and in-buil BEC set to 5.5V. I run 3 strandard coreless MKS servos and a BLS , standard size, on the tail. 'till now,no problems.
BUT,I'm still flying the prototype heli,with little differences like head dumpers,wich were remplaces and now are Red. All were corrected.My ESC is also different (the look),but I think the setting are still the same.
I've heard from Luca Brizzi,from Italian Gaui team, who use FBL on the X5 and 610
on cyclic that the stock ESC can be a bit to weak if you push it really hard,he recommends to use an external BEC.
But I think you can keep using it. Watch this video - here the same servo as yours were used!
I would take the bulk of 3 Align servos. They have very good specs,stong and fast.