Sanity check: Using the DS3231 as a clock source for uC
This PIC, and all other PIC18's that I know of, have an internal oscillator block.  There is no need for an external crystal, even if you didn't have the 32kHz output.  The only time you would need to use an external XTAL or clock would be:


To synchronize multiple devices to the same clock,
If you need a very specific frequency (for serial communications, etc), or
If you need more accuracy that the internal oscillator would provide.


You configure this INTOSC block by setting bits in the OSCCON register.  You can choose from eight values from 31kHz to 8MHz to run the system clock.

In the OSCCON register, set the SCS1 bit to use the internal oscillator block.  Then, set the IRCF2:IRCF0 bits to choose your frequency.  This picture is from Section 2.4.1 of the datasheet:



This is convenient because you can choose a slow frequency for lower power usage, and then increase it if you need to.

You could, of course, use the 32kHz RTC output, but it may just be unnecessary complexity.  Also, keep in mind that the PIC18F4550 has a different oscillator architecture than other PIC18's because of the way it clocks it's USB module.  This won't stop you from using the 32kHz signal, but it will require a different configuration that you may be used to.