I have a vga project that needs 31.5mhz. I'm not sure if the PLL is configurable enough to get me that or whether I should just run the whole chip of 31.5mhz oscillator for that project

The data sheet says it should be able to run at ~90MHz + an x3 divider, if you just yoink a copy of the design But it's an experimental / low confidence circuit so you should design in backup options like the mgmt area did

would there be any reason to not just run the management core at 31.5mhz?

I have 5 designs, and they have different clock requirements. So either I make the oscillator swappable, and run everything at the one speed. Or have an external swappable oscillator, or try the pll

what are you planning?