RE: BEC 5amp enoguh for 6 servos
Bottom Line...
Yes you should be fine.
The "Mighty Mini" draws around 1A at full stall, and that is something you'll NEVER see if you've set up your plane properly.
Typical current draw is around 300-400 under normal flight loads and much less during no loads. That 300-400 tends to be only momentary too.
The Castle DOES have a switching BEC, and you can use the logging feature to double check everything if you so desire.
6v is no problem with your setup.