Full travel on servos
Welcome to fixed wings, I don't know much about your Devo7 transmitter, but did see that it is for both fixed and rotary wing aircraft. I'm sure it can be adapted to move your servos both ways. I suspect it is in a heli mode, that you can disable by creating a new model memory that doesn't copy any of the heli commands.
When you say it moves only one way, do you mean one direction from center and back, or does it just drive the servo to one direction and stay there? Not knowing where you are flying, is the transmitter mode 1 or mode 2, throttle left stick or right stick?
I'm sure someone on here with more knowledge of helicopter setups will chime in with the proper answer, from what you say, I think your equipment will adapt easily. Your choice of a plane might be a bit of a challenge to master without a copilot. Good luck.