Pumpkin sour

The sour pros will speak up, but I believe even tolerant bacteria quits at like 10 IBUs. I would splite the batch pre-boil and do two boils with two different hop schedules. 20 to 30 IBUs schedule for pumpkin and under 10 IBUs schedule for sour.

 

20-30 IBU would definitely inhibit lacto/bacteria growth for a sour. You could collect your wort, sour it pre-boil, then do a full 60 min boil and add your hop additions to achieve 20-30 IBU. Then, you can add your cinnamon, nutmeg, all spice at flameout. From my experience, pumpkin in the mash doesn’t contribute much ‘pumpkin’ flavor. It’s more from the spices. You could technically do a sour pumpkin beer with out adding any pumpkin at all, but if you’re set on using pumpkin, that shouldn’t hurt the lacto souring pre-boil.

 

As @InVinoVeritas and @Lukass have suggested the hops will hurt you, and you've got to separate the batches. Everything else that's been said is true as well. There are many ways to do it, and I'm sure you'll figure it out, you really just need two kettles.

What exactly is your third generation sour yeast?

By the way, of all of the pumpkin beers I've tasted it was the sour ones that I've liked the best (but I'm a sour fan to begin with), and I'm actually planning this exact same thing except that mine will be more golden and done as a kettle sour with yogurt, and I'll sour the entire batch.

 

IMO, well attenuated and sour with pumpkin spices isn't a good flavor profile, but you do have some options available to you if you just want sour and pumpkin spice.

Are you just looking for something to do with the extra 5 gallons or do you have your heart set on making a sour pumpkin beer?