It depends if you want synchronus or asynchronus mode.
For asynchronus mode, the minimum speed is 2.44K Baud
For synchronus mode, the minimum speed is 39.06K Baud.
You need to implement some kind of encoding, Manchester Coding is the type usually employed. The limitations of the TX/RX modules prevent you sending straight RS232 - although if you invert it you can get away with it. This means either a hardware inverter between PIC's and modules, or using a software UART at each end - which would also allow you to use any speed you require.
Be aware that using Manchester Coding, or another type, will slow the data transfer down - something like 50%.
The 'resting' output from the UART is high, but the modules can't transmit a DC level - this corrupts the data, as the received ouput goes low (rather noisely) after a number of milliseconds.
As the UART polarity is fixed (it's designed to be used with a MAX232 or similar), you can't connect it to the modules without inverting it. It's also a bad idea to send plain RS232 over a radio link.
You just need to supply the encoder chip with the correct baud rate (and polarity - I would expect it to be simply pin to pin from the PIC?), it should be included in the application note with it.