With a newer GPS (eg ublox m8n) I find you can often get away with mounting it for forward flight, but this depends on your usage a little bit. Typically you intend to transition to fixed wing flight as soon as possible, and stay there for the majority of the time.
For servo position I generally try to get it as close as possible mechanically first. After that you can adjust the trim per PWM or control axis if needed.
Best flight mode for takeoff? Depends what you're trying to do, but bring it up slowly. Treat it like a multicopter in manual/stabilized mode first, do some tuning, then try altitude control mode, then position control mode.