First, let's see if it's even possible to use the Teensy as an I²C slave to the pi if they're connected through the Pi-Duino-Teensy board (I'll call it PDT from now on). According to the schematic, the Teensy's I²C pins (A4 - SDA and A5 - SCL) are connected to the pi's I²C pins (GPIO3 - SDA and GPIO5 - SCL) through 220 ohm resistors. I've never used inline resistors in I²C lines, but I don't think they will cause major problems.
Pull-ups? The PDT doesn't have any and iirc the pi has 1.8 kOhm on each I²C line. That's ok, I think.