The oscillators on the USB chips are extremely versatile.
You could do all the things you suggested.

With a 24Mhz crystal ...
You can have both the USB module and the CPU running from 48Mhz.
Or the CPU could be at 24, 12, 8 or 4 Mhz while the USB still gets 48.

You could also run the CPU from the internal oscillator at 8Mhz, while the USB module gets 48 Mhz from a 4Mhz crystal.

So many options you can't list them all.

I'd like tmr1 to have a 32Khz clock from it's internals without having to add on another xtal...
Now that ... you can't do.
But you can use the FOSC/4 from the CPU's clock.
Not as accurate as a Watch Crystal, especially when running at 48Mhz.

However, you'll need PBP 2.46 or higher to use 18F4550's with USB.
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