The UART, SPI, I2C, PWM, Analog, and Touch pins are all GPIO pins. Remember to use a current-limiting resistor if your 5V source can potentially provide more than 9 mA drive.
I(icio) -3 mA, +3 mA I/O pin DC current injection limit, single pin 
. All analog pins are internally clamped to VSS and VDD through ESD protection diodes. If VIN is greater than VAIO_MIN
(=VSS-0.3V) and VIN is less than VAIO_MAX(=VDD+0.3V) is observed, then there is no need to provide current limiting
resistors at the pads. If these limits cannot be observed then a current limiting resistor is required. The negative DC
injection current limiting resistor is calculated as R=(VAIO_MIN-VIN)/|IIC|. The positive injection current limiting resistor is
calcualted as R=(VIN-VAIO_MAX)/|IIC|. Select the larger of these two calculated resistances.
- Table 5.2.1, p. 11, Freescale Datasheet K20P64M50SF0
The Teensy 3.0 pin are not 5V tolerant.
You didn't specify which port on the AIOP you're using to communicate. I'm going to assume it's one of the serial ports.....
For the Teensy 3.0 TX to AIOP RX signal, you can connect the pins directly. Teensy 3.0 will output a 3.3 volt signal, which the chip on the AIOP can receive without any trouble.
For the APIO TX to Teensy 3.0 RX signal, you'll need to connect a series resistor. I'd recommend a value of 10K. The tiny delay caused by the resistor and pin capacitance is a non-issue for any reasonable baud rates like 115200 bps.
and 5v tolerance is essential before I can move to Teensy 3.0.
I can also tell you we're currently waiting on availability of a new chip from Freescale. I'm reluctant to give any exact release date, because Freescale has always been late releasing every other similar chip.