I'd still run 6 cells with the 6.5x5 but just keep your power on runs short until you are confident that your temperatures are OK.
What I would do is take off, do one 6 second run and land straight away then check your motor temperature. If OK, repeat with longer runs so that you gain confidence that you won't damage the motor.
What is safe for you might be different to mine because of differences in ambient temps, motor cooling (running a scoop?), length of WOT periods, battery voltage under load, length of cool down periods etc.
Don't stress, it'll be fun!