i am using the same thing. I actually just soldered up my ESC connections and motor connections last week. I do have the 6.5mm from Castle. I have found, what worked for me, is that I cut off the pre tinned leads. It seems that when they tinned them, they disturbed the wire enough that it won't fit in the connector.
SO, I cut their tinned lead off, and I tightened the wire in my fingers, and would test fit it with the 6.5mm connector to make sure it fit in it. I then retinned it, adding soldier to the tip to get it to flow, but trying not to disturb the tightness of the stranded wires of the ESC. I would then check the fit with the connector, and well for me, it fit.
I then would insert it over the connector, and apply heat. This is where an alligator clip comes in so handy, to hold the wire!! I would help hold the connector with a pair of micro needle nose, but if I had the alligator clip tilted just right, I wouldn't have to touch the connector. I'd then apply heat to the bottom of the connector, and would feed soldier into the hold in the bullet connector, and you can tell if you have enough in. For me, after that, I would look at where I put the wire in, if I could see soldier like it was about to come out, I know I got enough in it.
This is what worked for me. I did the same thing on my E5 and it worked great!! I'm using a soldering station I got from RadioShack, and have my heat turned up to 800 degrees. I should have taken a video of it, now that you mention it, as it's hard for me to explain how I do it.
Hope that helps!
