Teensy does not require a pullup resistor for hardware serial. I can't say for sure about PIC24, but generally most microcontrollers don't require pullup resistors.
But if the 2 boards are connected through a cable that might be unplugged, you may desire to have a pullup resistor to 3.3V on receive pins of both sides, so they won't hear any random interference when the cable is disconnected.
Of course the signals need to be 3.3V logic on both sides. Teensy 4.1 is not 5 volt tolerant. I don't know much about PIC24, but my limited understanding is it's one of the newer PIC architectures. The very old ones were all 5 volts.
Speed 500kBaud or higher.
Please for your own sanity, start with a slower baud rate just for the sake of getting the connection tested. Then increase once you have known-good communication.
Teensy 4.1 (32 bit dual-issue CPU) at 600 MHz with programming in C++ can usually handle 500 kbit/sec serial with ease. If your code does other work and ignores incoming serial data for short times, you can increase the buffer for incoming bytes with Serial2.addMemoryForRead(). While your program's design matters quite a lot, generally speaking this sort of speed is pretty easy on Teensy 4.1.
But what sort of speed you can reasonably expect from PIC24 (16 bit CPU) at 32 MHz running an interpreted language is difficult to say. Opinions vary regarding Forth. Some people are huge fans. If any of them are reading, maybe they can give you an idea of what sort of serial communication speed can be reasonably expected? But even with C language, the PIC24 side is likely to be the speed limiting part.
