Is tape strong enough? I'd think that it'd let go with time/forces to hold those flaps down. What about CA? is there a reason not to use CA?
I have used double-sided foam tape, known as "servo tape." It works pretty well if the mounting surface is flat and you are careful to clean the mounting surface to remove any grease or dirt. Molded foam or plastic planes may have some residual mold release on their surfaces, so it is important to clean it before attaching the servos.
If you use CA on a foam plane, be sure it is foam-safe CA. Regular CA may attack and melt the foam. To make it easier to remove the servo in the future, wrap the servo with a layer of tape so that you are gluing to the tape rather than directly to the plastic case of the servo.
I have not used CA for mounting servos, but I have used hot glue, which works well, in my experience. It holds firmly, yet you can usually remove the servo if necessary without damaging it. Hot glue works even if the mounting surface is slightly curved, whereas CA needs a flat surface for best results.
Also the servos come with the um ''lever parts'' attached. Do i just find the travel (about 180*) of the servos then center the ''lever/arm piece'' on the servo(90*)?
Those "lever parts" are called "servo arms." To center a servo, I connect it to the receiver, turn on the transmitter and receiver (ALWAYS turn on the transmitter first, then the receiver), and center the stick for that channel. Center the trim as well. This moves the servo to the center of its range of motion. Now you can attach the servo arm at the angle you want, usually (but not always!) 90 degrees to the body of the servo.